Fixed #11964 -- Added support for database check constraints.

29 files changed, 799 insertions, 39 deletions

diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index d5b142383a..582fa2dc1f 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -172,6 +172,7 @@ class BaseDatabaseFeatures:
 
     # Does it support CHECK constraints?
     supports_column_check_constraints = True
+    supports_table_check_constraints = True
 
     # Does the backend support 'pyformat' style ("... %(name)s ...", {'name': value})
     # parameter passing? Note this can be provided by the backend even if not
diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index ec2cf0e5c7..9608e95afb 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -63,7 +63,8 @@ class BaseDatabaseSchemaEditor:
     sql_rename_column = "ALTER TABLE %(table)s RENAME COLUMN %(old_column)s TO %(new_column)s"
     sql_update_with_default = "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
 
-    sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
+    sql_check = "CONSTRAINT %(name)s CHECK (%(check)s)"
+    sql_create_check = "ALTER TABLE %(table)s ADD %(check)s"
     sql_delete_check = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
 
     sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
@@ -299,10 +300,11 @@ class BaseDatabaseSchemaEditor:
         for fields in model._meta.unique_together:
             columns = [model._meta.get_field(field).column for field in fields]
             self.deferred_sql.append(self._create_unique_sql(model, columns))
+        constraints = [check.constraint_sql(model, self) for check in model._meta.constraints]
         # Make the table
         sql = self.sql_create_table % {
             "table": self.quote_name(model._meta.db_table),
-            "definition": ", ".join(column_sqls)
+            "definition": ", ".join((*column_sqls, *constraints)),
         }
         if model._meta.db_tablespace:
             tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
@@ -343,6 +345,14 @@ class BaseDatabaseSchemaEditor:
         """Remove an index from a model."""
         self.execute(index.remove_sql(model, self))
 
+    def add_constraint(self, model, constraint):
+        """Add a check constraint to a model."""
+        self.execute(constraint.create_sql(model, self))
+
+    def remove_constraint(self, model, constraint):
+        """Remove a check constraint from a model."""
+        self.execute(constraint.remove_sql(model, self))
+
     def alter_unique_together(self, model, old_unique_together, new_unique_together):
         """
         Deal with a model changing its unique_together. The input
@@ -752,11 +762,12 @@ class BaseDatabaseSchemaEditor:
             self.execute(
                 self.sql_create_check % {
                     "table": self.quote_name(model._meta.db_table),
-                    "name": self.quote_name(
-                        self._create_index_name(model._meta.db_table, [new_field.column], suffix="_check")
-                    ),
-                    "column": self.quote_name(new_field.column),
-                    "check": new_db_params['check'],
+                    "check": self.sql_check % {
+                        'name': self.quote_name(
+                            self._create_index_name(model._meta.db_table, [new_field.column], suffix='_check'),
+                        ),
+                        'check': new_db_params['check'],
+                    },
                 }
             )
         # Drop the default if we need to
diff --git a/django/db/backends/mysql/features.py b/django/db/backends/mysql/features.py
index ec80c6e54e..b50513d779 100644
--- a/django/db/backends/mysql/features.py
+++ b/django/db/backends/mysql/features.py
@@ -26,6 +26,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     can_release_savepoints = True
     atomic_transactions = False
     supports_column_check_constraints = False
+    supports_table_check_constraints = False
     can_clone_databases = True
     supports_temporal_subtraction = True
     supports_select_intersection = False
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index b27d39d732..8710e9d0e2 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -126,7 +126,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         else:
             super().alter_field(model, old_field, new_field, strict=strict)
 
-    def _remake_table(self, model, create_field=None, delete_field=None, alter_field=None):
+    def _remake_table(self, model, create_field=None, delete_field=None, alter_field=None,
+                      add_constraint=None, remove_constraint=None):
         """
         Shortcut to transform a model from old_model into new_model
 
@@ -222,6 +223,15 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 if delete_field.name not in index.fields
             ]
 
+        constraints = list(model._meta.constraints)
+        if add_constraint:
+            constraints.append(add_constraint)
+        if remove_constraint:
+            constraints = [
+                constraint for constraint in constraints
+                if remove_constraint.name != constraint.name
+            ]
+
         # Construct a new model for the new state
         meta_contents = {
             'app_label': model._meta.app_label,
@@ -229,6 +239,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             'unique_together': unique_together,
             'index_together': index_together,
             'indexes': indexes,
+            'constraints': constraints,
             'apps': apps,
         }
         meta = type("Meta", (), meta_contents)
@@ -362,3 +373,9 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         ))
         # Delete the old through table
         self.delete_model(old_field.remote_field.through)
+
+    def add_constraint(self, model, constraint):
+        self._remake_table(model, add_constraint=constraint)
+
+    def remove_constraint(self, model, constraint):
+        self._remake_table(model, remove_constraint=constraint)
diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index f32d4af9eb..bf9a45530a 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -122,6 +122,7 @@ class MigrationAutodetector:
         # resolve dependencies caused by M2Ms and FKs.
         self.generated_operations = {}
         self.altered_indexes = {}
+        self.altered_constraints = {}
 
         # Prepare some old/new state and model lists, separating
         # proxy models and ignoring unmigrated apps.
@@ -175,7 +176,9 @@ class MigrationAutodetector:
         # This avoids the same computation in generate_removed_indexes()
         # and generate_added_indexes().
         self.create_altered_indexes()
+        self.create_altered_constraints()
         # Generate index removal operations before field is removed
+        self.generate_removed_constraints()
         self.generate_removed_indexes()
         # Generate field operations
         self.generate_renamed_fields()
@@ -185,6 +188,7 @@ class MigrationAutodetector:
         self.generate_altered_unique_together()
         self.generate_altered_index_together()
         self.generate_added_indexes()
+        self.generate_added_constraints()
         self.generate_altered_db_table()
         self.generate_altered_order_with_respect_to()
 
@@ -533,6 +537,7 @@ class MigrationAutodetector:
                     related_fields[field.name] = field
             # Are there indexes/unique|index_together to defer?
             indexes = model_state.options.pop('indexes')
+            constraints = model_state.options.pop('constraints')
             unique_together = model_state.options.pop('unique_together', None)
             index_together = model_state.options.pop('index_together', None)
             order_with_respect_to = model_state.options.pop('order_with_respect_to', None)
@@ -601,6 +606,15 @@ class MigrationAutodetector:
                     ),
                     dependencies=related_dependencies,
                 )
+            for constraint in constraints:
+                self.add_operation(
+                    app_label,
+                    operations.AddConstraint(
+                        model_name=model_name,
+                        constraint=constraint,
+                    ),
+                    dependencies=related_dependencies,
+                )
             if unique_together:
                 self.add_operation(
                     app_label,
@@ -997,6 +1011,46 @@ class MigrationAutodetector:
                     )
                 )
 
+    def create_altered_constraints(self):
+        option_name = operations.AddConstraint.option_name
+        for app_label, model_name in sorted(self.kept_model_keys):
+            old_model_name = self.renamed_models.get((app_label, model_name), model_name)
+            old_model_state = self.from_state.models[app_label, old_model_name]
+            new_model_state = self.to_state.models[app_label, model_name]
+
+            old_constraints = old_model_state.options[option_name]
+            new_constraints = new_model_state.options[option_name]
+            add_constraints = [c for c in new_constraints if c not in old_constraints]
+            rem_constraints = [c for c in old_constraints if c not in new_constraints]
+
+            self.altered_constraints.update({
+                (app_label, model_name): {
+                    'added_constraints': add_constraints, 'removed_constraints': rem_constraints,
+                }
+            })
+
+    def generate_added_constraints(self):
+        for (app_label, model_name), alt_constraints in self.altered_constraints.items():
+            for constraint in alt_constraints['added_constraints']:
+                self.add_operation(
+                    app_label,
+                    operations.AddConstraint(
+                        model_name=model_name,
+                        constraint=constraint,
+                    )
+                )
+
+    def generate_removed_constraints(self):
+        for (app_label, model_name), alt_constraints in self.altered_constraints.items():
+            for constraint in alt_constraints['removed_constraints']:
+                self.add_operation(
+                    app_label,
+                    operations.RemoveConstraint(
+                        model_name=model_name,
+                        name=constraint.name,
+                    )
+                )
+
     def _get_dependencies_for_foreign_key(self, field):
         # Account for FKs to swappable models
         swappable_setting = getattr(field, 'swappable_setting', None)
diff --git a/django/db/migrations/operations/__init__.py b/django/db/migrations/operations/__init__.py
index 894f2ab9c5..119c955868 100644
--- a/django/db/migrations/operations/__init__.py
+++ b/django/db/migrations/operations/__init__.py
@@ -1,8 +1,9 @@
 from .fields import AddField, AlterField, RemoveField, RenameField
 from .models import (
-    AddIndex, AlterIndexTogether, AlterModelManagers, AlterModelOptions,
-    AlterModelTable, AlterOrderWithRespectTo, AlterUniqueTogether, CreateModel,
-    DeleteModel, RemoveIndex, RenameModel,
+    AddConstraint, AddIndex, AlterIndexTogether, AlterModelManagers,
+    AlterModelOptions, AlterModelTable, AlterOrderWithRespectTo,
+    AlterUniqueTogether, CreateModel, DeleteModel, RemoveConstraint,
+    RemoveIndex, RenameModel,
 )
 from .special import RunPython, RunSQL, SeparateDatabaseAndState
 
@@ -10,6 +11,7 @@ __all__ = [
     'CreateModel', 'DeleteModel', 'AlterModelTable', 'AlterUniqueTogether',
     'RenameModel', 'AlterIndexTogether', 'AlterModelOptions', 'AddIndex',
     'RemoveIndex', 'AddField', 'RemoveField', 'AlterField', 'RenameField',
+    'AddConstraint', 'RemoveConstraint',
     'SeparateDatabaseAndState', 'RunSQL', 'RunPython',
     'AlterOrderWithRespectTo', 'AlterModelManagers',
 ]
diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index 857981bcb8..bd1c66a01d 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -822,3 +822,72 @@ class RemoveIndex(IndexOperation):
 
     def describe(self):
         return 'Remove index %s from %s' % (self.name, self.model_name)
+
+
+class AddConstraint(IndexOperation):
+    option_name = 'constraints'
+
+    def __init__(self, model_name, constraint):
+        self.model_name = model_name
+        self.constraint = constraint
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        constraints = list(model_state.options[self.option_name])
+        constraints.append(self.constraint)
+        model_state.options[self.option_name] = constraints
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.add_constraint(model, self.constraint)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.remove_constraint(model, self.constraint)
+
+    def deconstruct(self):
+        return self.__class__.__name__, [], {
+            'model_name': self.model_name,
+            'constraint': self.constraint,
+        }
+
+    def describe(self):
+        return 'Create constraint %s on model %s' % (self.constraint.name, self.model_name)
+
+
+class RemoveConstraint(IndexOperation):
+    option_name = 'constraints'
+
+    def __init__(self, model_name, name):
+        self.model_name = model_name
+        self.name = name
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        constraints = model_state.options[self.option_name]
+        model_state.options[self.option_name] = [c for c in constraints if c.name != self.name]
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            from_model_state = from_state.models[app_label, self.model_name_lower]
+            constraint = from_model_state.get_constraint_by_name(self.name)
+            schema_editor.remove_constraint(model, constraint)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            to_model_state = to_state.models[app_label, self.model_name_lower]
+            constraint = to_model_state.get_constraint_by_name(self.name)
+            schema_editor.add_constraint(model, constraint)
+
+    def deconstruct(self):
+        return self.__class__.__name__, [], {
+            'model_name': self.model_name,
+            'name': self.name,
+        }
+
+    def describe(self):
+        return 'Remove constraint %s from model %s' % (self.name, self.model_name)
diff --git a/django/db/migrations/state.py b/django/db/migrations/state.py
index f41d1edf2c..ea2db0e5af 100644
--- a/django/db/migrations/state.py
+++ b/django/db/migrations/state.py
@@ -362,6 +362,7 @@ class ModelState:
         self.fields = fields
         self.options = options or {}
         self.options.setdefault('indexes', [])
+        self.options.setdefault('constraints', [])
         self.bases = bases or (models.Model,)
         self.managers = managers or []
         # Sanity-check that fields is NOT a dict. It must be ordered.
@@ -445,6 +446,8 @@ class ModelState:
                         if not index.name:
                             index.set_name_with_model(model)
                     options['indexes'] = indexes
+                elif name == 'constraints':
+                    options['constraints'] = [con.clone() for con in model._meta.constraints]
                 else:
                     options[name] = model._meta.original_attrs[name]
         # If we're ignoring relationships, remove all field-listing model
@@ -585,6 +588,12 @@ class ModelState:
                 return index
         raise ValueError("No index named %s on model %s" % (name, self.name))
 
+    def get_constraint_by_name(self, name):
+        for constraint in self.options['constraints']:
+            if constraint.name == name:
+                return constraint
+        raise ValueError('No constraint named %s on model %s' % (name, self.name))
+
     def __repr__(self):
         return "<%s: '%s.%s'>" % (self.__class__.__name__, self.app_label, self.name)
 
diff --git a/django/db/models/__init__.py b/django/db/models/__init__.py
index 27c67c7fc4..79b175c1d5 100644
--- a/django/db/models/__init__.py
+++ b/django/db/models/__init__.py
@@ -2,6 +2,8 @@ from django.core.exceptions import ObjectDoesNotExist
 from django.db.models import signals
 from django.db.models.aggregates import *  # NOQA
 from django.db.models.aggregates import __all__ as aggregates_all
+from django.db.models.constraints import *  # NOQA
+from django.db.models.constraints import __all__ as constraints_all
 from django.db.models.deletion import (
     CASCADE, DO_NOTHING, PROTECT, SET, SET_DEFAULT, SET_NULL, ProtectedError,
 )
@@ -30,7 +32,7 @@ from django.db.models.fields.related import (  # isort:skip
 )
 
 
-__all__ = aggregates_all + fields_all + indexes_all
+__all__ = aggregates_all + constraints_all + fields_all + indexes_all
 __all__ += [
     'ObjectDoesNotExist', 'signals',
     'CASCADE', 'DO_NOTHING', 'PROTECT', 'SET', 'SET_DEFAULT', 'SET_NULL',
diff --git a/django/db/models/base.py b/django/db/models/base.py
index 251825991f..3574f7f676 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -16,6 +16,7 @@ from django.db import (
     connections, router, transaction,
 )
 from django.db.models.constants import LOOKUP_SEP
+from django.db.models.constraints import CheckConstraint
 from django.db.models.deletion import CASCADE, Collector
 from django.db.models.fields.related import (
     ForeignObjectRel, OneToOneField, lazy_related_operation, resolve_relation,
@@ -1201,6 +1202,7 @@ class Model(metaclass=ModelBase):
                 *cls._check_unique_together(),
                 *cls._check_indexes(),
                 *cls._check_ordering(),
+                *cls._check_constraints(),
             ]
 
         return errors
@@ -1699,6 +1701,29 @@ class Model(metaclass=ModelBase):
 
         return errors
 
+    @classmethod
+    def _check_constraints(cls):
+        errors = []
+        for db in settings.DATABASES:
+            if not router.allow_migrate_model(db, cls):
+                continue
+            connection = connections[db]
+            if connection.features.supports_table_check_constraints:
+                continue
+            if any(isinstance(constraint, CheckConstraint) for constraint in cls._meta.constraints):
+                errors.append(
+                    checks.Warning(
+                        '%s does not support check constraints.' % connection.display_name,
+                        hint=(
+                            "A constraint won't be created. Silence this "
+                            "warning if you don't care about it."
+                        ),
+                        obj=cls,
+                        id='models.W027',
+                    )
+                )
+        return errors
+
 
 ############################################
 # HELPER FUNCTIONS (CURRIED MODEL METHODS) #
diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
new file mode 100644
index 0000000000..fe99f8310d
--- /dev/null
+++ b/django/db/models/constraints.py
@@ -0,0 +1,54 @@
+from django.db.models.sql.query import Query
+
+__all__ = ['CheckConstraint']
+
+
+class CheckConstraint:
+    def __init__(self, constraint, name):
+        self.constraint = constraint
+        self.name = name
+
+    def constraint_sql(self, model, schema_editor):
+        query = Query(model)
+        where = query.build_where(self.constraint)
+        connection = schema_editor.connection
+        compiler = connection.ops.compiler('SQLCompiler')(query, connection, 'default')
+        sql, params = where.as_sql(compiler, connection)
+        params = tuple(schema_editor.quote_value(p) for p in params)
+        return schema_editor.sql_check % {
+            'name': schema_editor.quote_name(self.name),
+            'check': sql % params,
+        }
+
+    def create_sql(self, model, schema_editor):
+        sql = self.constraint_sql(model, schema_editor)
+        return schema_editor.sql_create_check % {
+            'table': schema_editor.quote_name(model._meta.db_table),
+            'check': sql,
+        }
+
+    def remove_sql(self, model, schema_editor):
+        quote_name = schema_editor.quote_name
+        return schema_editor.sql_delete_check % {
+            'table': quote_name(model._meta.db_table),
+            'name': quote_name(self.name),
+        }
+
+    def __repr__(self):
+        return "<%s: constraint='%s' name='%s'>" % (self.__class__.__name__, self.constraint, self.name)
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, CheckConstraint) and
+            self.name == other.name and
+            self.constraint == other.constraint
+        )
+
+    def deconstruct(self):
+        path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
+        path = path.replace('django.db.models.constraints', 'django.db.models')
+        return (path, (), {'constraint': self.constraint, 'name': self.name})
+
+    def clone(self):
+        _, args, kwargs = self.deconstruct()
+        return self.__class__(*args, **kwargs)
diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 219485750f..86ce77daa2 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -505,8 +505,9 @@ class F(Combinable):
     def __repr__(self):
         return "{}({})".format(self.__class__.__name__, self.name)
 
-    def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
-        return query.resolve_ref(self.name, allow_joins, reuse, summarize)
+    def resolve_expression(self, query=None, allow_joins=True, reuse=None,
+                           summarize=False, for_save=False, simple_col=False):
+        return query.resolve_ref(self.name, allow_joins, reuse, summarize, simple_col)
 
     def asc(self, **kwargs):
         return OrderBy(self, **kwargs)
@@ -542,7 +543,8 @@ class ResolvedOuterRef(F):
 
 
 class OuterRef(F):
-    def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
+    def resolve_expression(self, query=None, allow_joins=True, reuse=None,
+                           summarize=False, for_save=False, simple_col=False):
         if isinstance(self.name, self.__class__):
             return self.name
         return ResolvedOuterRef(self.name)
@@ -746,6 +748,40 @@ class Col(Expression):
                 self.target.get_db_converters(connection))
 
 
+class SimpleCol(Expression):
+    """
+    Represents the SQL of a column name without the table name.
+
+    This variant of Col doesn't include the table name (or an alias) to
+    avoid a syntax error in check constraints.
+    """
+    contains_column_references = True
+
+    def __init__(self, target, output_field=None):
+        if output_field is None:
+            output_field = target
+        super().__init__(output_field=output_field)
+        self.target = target
+
+    def __repr__(self):
+        return '{}({})'.format(self.__class__.__name__, self.target)
+
+    def as_sql(self, compiler, connection):
+        qn = compiler.quote_name_unless_alias
+        return qn(self.target.column), []
+
+    def get_group_by_cols(self):
+        return [self]
+
+    def get_db_converters(self, connection):
+        if self.target == self.output_field:
+            return self.output_field.get_db_converters(connection)
+        return (
+            self.output_field.get_db_converters(connection) +
+            self.target.get_db_converters(connection)
+        )
+
+
 class Ref(Expression):
     """
     Reference to column alias of the query. For example, Ref('sum_cost') in
diff --git a/django/db/models/options.py b/django/db/models/options.py
index c0c925375f..98bd2f0064 100644
--- a/django/db/models/options.py
+++ b/django/db/models/options.py
@@ -32,7 +32,7 @@ DEFAULT_NAMES = (
     'auto_created', 'index_together', 'apps', 'default_permissions',
     'select_on_save', 'default_related_name', 'required_db_features',
     'required_db_vendor', 'base_manager_name', 'default_manager_name',
-    'indexes',
+    'indexes', 'constraints',
     # For backwards compatibility with Django 1.11. RemovedInDjango30Warning
     'manager_inheritance_from_future',
 )
@@ -89,6 +89,7 @@ class Options:
         self.ordering = []
         self._ordering_clash = False
         self.indexes = []
+        self.constraints = []
         self.unique_together = []
         self.index_together = []
         self.select_on_save = False
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 01ff007eda..2a3510f992 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -18,7 +18,7 @@ from django.core.exceptions import (
 from django.db import DEFAULT_DB_ALIAS, NotSupportedError, connections
 from django.db.models.aggregates import Count
 from django.db.models.constants import LOOKUP_SEP
-from django.db.models.expressions import Col, Ref
+from django.db.models.expressions import Col, F, Ref, SimpleCol
 from django.db.models.fields import Field
 from django.db.models.fields.related_lookups import MultiColSource
 from django.db.models.lookups import Lookup
@@ -62,6 +62,12 @@ JoinInfo = namedtuple(
 )
 
 
+def _get_col(target, field, alias, simple_col):
+    if simple_col:
+        return SimpleCol(target, field)
+    return target.get_col(alias, field)
+
+
 class RawQuery:
     """A single raw SQL query."""
 
@@ -1011,15 +1017,24 @@ class Query:
     def as_sql(self, compiler, connection):
         return self.get_compiler(connection=connection).as_sql()
 
-    def resolve_lookup_value(self, value, can_reuse, allow_joins):
+    def resolve_lookup_value(self, value, can_reuse, allow_joins, simple_col):
         if hasattr(value, 'resolve_expression'):
-            value = value.resolve_expression(self, reuse=can_reuse, allow_joins=allow_joins)
+            kwargs = {'reuse': can_reuse, 'allow_joins': allow_joins}
+            if isinstance(value, F):
+                kwargs['simple_col'] = simple_col
+            value = value.resolve_expression(self, **kwargs)
         elif isinstance(value, (list, tuple)):
             # The items of the iterable may be expressions and therefore need
             # to be resolved independently.
             for sub_value in value:
                 if hasattr(sub_value, 'resolve_expression'):
-                    sub_value.resolve_expression(self, reuse=can_reuse, allow_joins=allow_joins)
+                    if isinstance(sub_value, F):
+                        sub_value.resolve_expression(
+                            self, reuse=can_reuse, allow_joins=allow_joins,
+                            simple_col=simple_col,
+                        )
+                    else:
+                        sub_value.resolve_expression(self, reuse=can_reuse, allow_joins=allow_joins)
         return value
 
     def solve_lookup_type(self, lookup):
@@ -1133,7 +1148,7 @@ class Query:
 
     def build_filter(self, filter_expr, branch_negated=False, current_negated=False,
                      can_reuse=None, allow_joins=True, split_subq=True,
-                     reuse_with_filtered_relation=False):
+                     reuse_with_filtered_relation=False, simple_col=False):
         """
         Build a WhereNode for a single filter clause but don't add it
         to this Query. Query.add_q() will then add this filter to the where
@@ -1179,7 +1194,7 @@ class Query:
             raise FieldError("Joined field references are not permitted in this query")
 
         pre_joins = self.alias_refcount.copy()
-        value = self.resolve_lookup_value(value, can_reuse, allow_joins)
+        value = self.resolve_lookup_value(value, can_reuse, allow_joins, simple_col)
         used_joins = {k for k, v in self.alias_refcount.items() if v > pre_joins.get(k, 0)}
 
         clause = self.where_class()
@@ -1222,11 +1237,11 @@ class Query:
             if num_lookups > 1:
                 raise FieldError('Related Field got invalid lookup: {}'.format(lookups[0]))
             if len(targets) == 1:
-                col = targets[0].get_col(alias, join_info.final_field)
+                col = _get_col(targets[0], join_info.final_field, alias, simple_col)
             else:
                 col = MultiColSource(alias, targets, join_info.targets, join_info.final_field)
         else:
-            col = targets[0].get_col(alias, join_info.final_field)
+            col = _get_col(targets[0], join_info.final_field, alias, simple_col)
 
         condition = self.build_lookup(lookups, col, value)
         lookup_type = condition.lookup_name
@@ -1248,7 +1263,8 @@ class Query:
                 #   <=>
                 # NOT (col IS NOT NULL AND col = someval).
                 lookup_class = targets[0].get_lookup('isnull')
-                clause.add(lookup_class(targets[0].get_col(alias, join_info.targets[0]), False), AND)
+                col = _get_col(targets[0], join_info.targets[0], alias, simple_col)
+                clause.add(lookup_class(col, False), AND)
         return clause, used_joins if not require_outer else ()
 
     def add_filter(self, filter_clause):
@@ -1271,8 +1287,12 @@ class Query:
             self.where.add(clause, AND)
         self.demote_joins(existing_inner)
 
+    def build_where(self, q_object):
+        return self._add_q(q_object, used_aliases=set(), allow_joins=False, simple_col=True)[0]
+
     def _add_q(self, q_object, used_aliases, branch_negated=False,
-               current_negated=False, allow_joins=True, split_subq=True):
+               current_negated=False, allow_joins=True, split_subq=True,
+               simple_col=False):
         """Add a Q-object to the current filter."""
         connector = q_object.connector
         current_negated = current_negated ^ q_object.negated
@@ -1290,7 +1310,7 @@ class Query:
                 child_clause, needed_inner = self.build_filter(
                     child, can_reuse=used_aliases, branch_negated=branch_negated,
                     current_negated=current_negated, allow_joins=allow_joins,
-                    split_subq=split_subq,
+                    split_subq=split_subq, simple_col=simple_col,
                 )
                 joinpromoter.add_votes(needed_inner)
             if child_clause:
@@ -1559,7 +1579,7 @@ class Query:
             self.unref_alias(joins.pop())
         return targets, joins[-1], joins
 
-    def resolve_ref(self, name, allow_joins=True, reuse=None, summarize=False):
+    def resolve_ref(self, name, allow_joins=True, reuse=None, summarize=False, simple_col=False):
         if not allow_joins and LOOKUP_SEP in name:
             raise FieldError("Joined field references are not permitted in this query")
         if name in self.annotations:
@@ -1580,7 +1600,7 @@ class Query:
                                  "isn't supported")
             if reuse is not None:
                 reuse.update(join_list)
-            col = targets[0].get_col(join_list[-1], join_info.targets[0])
+            col = _get_col(targets[0], join_info.targets[0], join_list[-1], simple_col)
             return col
 
     def split_exclude(self, filter_expr, can_reuse, names_with_path):
diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index b0d7dfc066..ee16ebde54 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -297,6 +297,7 @@ Models
   field accessor.
 * **models.E026**: The model cannot have more than one field with
   ``primary_key=True``.
+* **models.W027**: ``<database>`` does not support check constraints.
 
 Security
 --------
diff --git a/docs/ref/migration-operations.txt b/docs/ref/migration-operations.txt
index b45134b46d..c117145fde 100644
--- a/docs/ref/migration-operations.txt
+++ b/docs/ref/migration-operations.txt
@@ -207,6 +207,25 @@ Creates an index in the database table for the model with ``model_name``.
 
 Removes the index named ``name`` from the model with ``model_name``.
 
+``AddConstraint``
+-----------------
+
+.. class:: AddConstraint(model_name, constraint)
+
+.. versionadded:: 2.2
+
+Creates a constraint in the database table for the model with ``model_name``.
+``constraint`` is an instance of :class:`~django.db.models.CheckConstraint`.
+
+``RemoveConstraint``
+--------------------
+
+.. class:: RemoveConstraint(model_name, name)
+
+.. versionadded:: 2.2
+
+Removes the constraint named ``name`` from the model with ``model_name``.
+
 Special Operations
 ==================
 
diff --git a/docs/ref/models/check-constraints.txt b/docs/ref/models/check-constraints.txt
new file mode 100644
index 0000000000..29681d7ebc
--- /dev/null
+++ b/docs/ref/models/check-constraints.txt
@@ -0,0 +1,46 @@
+===========================
+Check constraints reference
+===========================
+
+.. module:: django.db.models.constraints
+
+.. currentmodule:: django.db.models
+
+.. versionadded:: 2.2
+
+The ``CheckConstraint`` class creates database check constraints. They are
+added in the model :attr:`Meta.constraints
+<django.db.models.Options.constraints>` option. This document
+explains the API references of :class:`CheckConstraint`.
+
+.. admonition:: Referencing built-in constraints
+
+    Constraints are defined in ``django.db.models.constraints``, but for
+    convenience they're imported into :mod:`django.db.models`. The standard
+    convention is to use ``from django.db import models`` and refer to the
+    constraints as ``models.CheckConstraint``.
+
+``CheckConstraint`` options
+===========================
+
+.. class:: CheckConstraint(constraint, name)
+
+    Creates a check constraint in the database.
+
+``constraint``
+--------------
+
+.. attribute:: CheckConstraint.constraint
+
+A :class:`Q` object that specifies the condition you want the constraint to
+enforce.
+
+For example ``CheckConstraint(Q(age__gte=18), 'age_gte_18')`` ensures the age
+field is never less than 18.
+
+``name``
+--------
+
+.. attribute:: CheckConstraint.name
+
+The name of the constraint.
diff --git a/docs/ref/models/index.txt b/docs/ref/models/index.txt
index d731ee37dc..c3aa5a718a 100644
--- a/docs/ref/models/index.txt
+++ b/docs/ref/models/index.txt
@@ -9,6 +9,7 @@ Model API reference. For introductory material, see :doc:`/topics/db/models`.
 
    fields
    indexes
+   check-constraints
    meta
    relations
    class
diff --git a/docs/ref/models/options.txt b/docs/ref/models/options.txt
index cea73eb67f..246f3e7d9a 100644
--- a/docs/ref/models/options.txt
+++ b/docs/ref/models/options.txt
@@ -451,6 +451,26 @@ Django quotes column and table names behind the scenes.
 
         index_together = ["pub_date", "deadline"]
 
+``constraints``
+---------------
+
+.. attribute:: Options.constraints
+
+    .. versionadded:: 2.2
+
+    A list of :doc:`constraints </ref/models/check-constraints>` that you want
+    to define on the model::
+
+        from django.db import models
+
+        class Customer(models.Model):
+            age = models.IntegerField()
+
+            class Meta:
+                constraints = [
+                    models.CheckConstraint(models.Q(age__gte=18), 'age_gte_18'),
+                ]
+
 ``verbose_name``
 ----------------
 
diff --git a/docs/releases/2.2.txt b/docs/releases/2.2.txt
index 0fdd57e340..29a6f54f9d 100644
--- a/docs/releases/2.2.txt
+++ b/docs/releases/2.2.txt
@@ -30,6 +30,13 @@ officially support the latest release of each series.
 What's new in Django 2.2
 ========================
 
+Check Constraints
+-----------------
+
+The new :class:`~django.db.models.CheckConstraint` class enables adding custom
+database constraints. Constraints are added to models using the
+:attr:`Meta.constraints <django.db.models.Options.constraints>` option.
+
 Minor features
 --------------
 
@@ -213,7 +220,9 @@ Backwards incompatible changes in 2.2
 Database backend API
 --------------------
 
-* ...
+* Third-party database backends must implement support for table check
+  constraints or set ``DatabaseFeatures.supports_table_check_constraints`` to
+  ``False``.
 
 :mod:`django.contrib.gis`
 -------------------------
diff --git a/tests/constraints/__init__.py b/tests/constraints/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tests/constraints/__init__.py
diff --git a/tests/constraints/models.py b/tests/constraints/models.py
new file mode 100644
index 0000000000..de49fa2765
--- /dev/null
+++ b/tests/constraints/models.py
@@ -0,0 +1,15 @@
+from django.db import models
+
+
+class Product(models.Model):
+    name = models.CharField(max_length=255)
+    price = models.IntegerField()
+    discounted_price = models.IntegerField()
+
+    class Meta:
+        constraints = [
+            models.CheckConstraint(
+                models.Q(price__gt=models.F('discounted_price')),
+                'price_gt_discounted_price'
+            )
+        ]
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
new file mode 100644
index 0000000000..19573dffa1
--- /dev/null
+++ b/tests/constraints/tests.py
@@ -0,0 +1,30 @@
+from django.db import IntegrityError, models
+from django.test import TestCase, skipUnlessDBFeature
+
+from .models import Product
+
+
+class CheckConstraintTests(TestCase):
+    def test_repr(self):
+        constraint = models.Q(price__gt=models.F('discounted_price'))
+        name = 'price_gt_discounted_price'
+        check = models.CheckConstraint(constraint, name)
+        self.assertEqual(
+            repr(check),
+            "<CheckConstraint: constraint='{}' name='{}'>".format(constraint, name),
+        )
+
+    def test_deconstruction(self):
+        constraint = models.Q(price__gt=models.F('discounted_price'))
+        name = 'price_gt_discounted_price'
+        check = models.CheckConstraint(constraint, name)
+        path, args, kwargs = check.deconstruct()
+        self.assertEqual(path, 'django.db.models.CheckConstraint')
+        self.assertEqual(args, ())
+        self.assertEqual(kwargs, {'constraint': constraint, 'name': name})
+
+    @skipUnlessDBFeature('supports_table_check_constraints')
+    def test_model_constraint(self):
+        Product.objects.create(name='Valid', price=10, discounted_price=5)
+        with self.assertRaises(IntegrityError):
+            Product.objects.create(name='Invalid', price=10, discounted_price=20)
diff --git a/tests/invalid_models_tests/test_models.py b/tests/invalid_models_tests/test_models.py
index 19ec21c9ae..9dd2fd1f06 100644
--- a/tests/invalid_models_tests/test_models.py
+++ b/tests/invalid_models_tests/test_models.py
@@ -1,10 +1,10 @@
 import unittest
 
 from django.conf import settings
-from django.core.checks import Error
+from django.core.checks import Error, Warning
 from django.core.checks.model_checks import _check_lazy_references
 from django.core.exceptions import ImproperlyConfigured
-from django.db import connections, models
+from django.db import connection, connections, models
 from django.db.models.signals import post_init
 from django.test import SimpleTestCase
 from django.test.utils import isolate_apps, override_settings
@@ -972,3 +972,26 @@ class OtherModelTests(SimpleTestCase):
                 id='signals.E001',
             ),
         ])
+
+
+@isolate_apps('invalid_models_tests')
+class ConstraintsTests(SimpleTestCase):
+    def test_check_constraints(self):
+        class Model(models.Model):
+            age = models.IntegerField()
+
+            class Meta:
+                constraints = [models.CheckConstraint(models.Q(age__gte=18), 'is_adult')]
+
+        errors = Model.check()
+        warn = Warning(
+            '%s does not support check constraints.' % connection.display_name,
+            hint=(
+                "A constraint won't be created. Silence this warning if you "
+                "don't care about it."
+            ),
+            obj=Model,
+            id='models.W027',
+        )
+        expected = [] if connection.features.supports_table_check_constraints else [warn, warn]
+        self.assertCountEqual(errors, expected)
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index fd1bc383b9..1fde1ba466 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -61,6 +61,12 @@ class AutodetectorTests(TestCase):
         ("id", models.AutoField(primary_key=True)),
         ("name", models.CharField(max_length=200, default='Ada Lovelace')),
     ])
+    author_name_check_constraint = ModelState("testapp", "Author", [
+        ("id", models.AutoField(primary_key=True)),
+        ("name", models.CharField(max_length=200)),
+    ],
+        {'constraints': [models.CheckConstraint(models.Q(name__contains='Bob'), 'name_contains_bob')]},
+    )
     author_dates_of_birth_auto_now = ModelState("testapp", "Author", [
         ("id", models.AutoField(primary_key=True)),
         ("date_of_birth", models.DateField(auto_now=True)),
@@ -1389,6 +1395,40 @@ class AutodetectorTests(TestCase):
         added_index = models.Index(fields=['title', 'author'], name='book_author_title_idx')
         self.assertOperationAttributes(changes, 'otherapp', 0, 1, model_name='book', index=added_index)
 
+    def test_create_model_with_check_constraint(self):
+        """Test creation of new model with constraints already defined."""
+        author = ModelState('otherapp', 'Author', [
+            ('id', models.AutoField(primary_key=True)),
+            ('name', models.CharField(max_length=200)),
+        ], {'constraints': [models.CheckConstraint(models.Q(name__contains='Bob'), 'name_contains_bob')]})
+        changes = self.get_changes([], [author])
+        added_constraint = models.CheckConstraint(models.Q(name__contains='Bob'), 'name_contains_bob')
+        # Right number of migrations?
+        self.assertEqual(len(changes['otherapp']), 1)
+        # Right number of actions?
+        migration = changes['otherapp'][0]
+        self.assertEqual(len(migration.operations), 2)
+        # Right actions order?
+        self.assertOperationTypes(changes, 'otherapp', 0, ['CreateModel', 'AddConstraint'])
+        self.assertOperationAttributes(changes, 'otherapp', 0, 0, name='Author')
+        self.assertOperationAttributes(changes, 'otherapp', 0, 1, model_name='author', constraint=added_constraint)
+
+    def test_add_constraints(self):
+        """Test change detection of new constraints."""
+        changes = self.get_changes([self.author_name], [self.author_name_check_constraint])
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ['AddConstraint'])
+        added_constraint = models.CheckConstraint(models.Q(name__contains='Bob'), 'name_contains_bob')
+        self.assertOperationAttributes(changes, 'testapp', 0, 0, model_name='author', constraint=added_constraint)
+
+    def test_remove_constraints(self):
+        """Test change detection of removed constraints."""
+        changes = self.get_changes([self.author_name_check_constraint], [self.author_name])
+        # Right number/type of migrations?
+        self.assertNumberMigrations(changes, 'testapp', 1)
+        self.assertOperationTypes(changes, 'testapp', 0, ['RemoveConstraint'])
+        self.assertOperationAttributes(changes, 'testapp', 0, 0, model_name='author', name='name_contains_bob')
+
     def test_add_foo_together(self):
         """Tests index/unique_together detection."""
         changes = self.get_changes([self.author_empty, self.book], [self.author_empty, self.book_foo_together])
@@ -1520,7 +1560,7 @@ class AutodetectorTests(TestCase):
         self.assertNumberMigrations(changes, "testapp", 1)
         self.assertOperationTypes(changes, "testapp", 0, ["CreateModel"])
         self.assertOperationAttributes(
-            changes, "testapp", 0, 0, name="AuthorProxy", options={"proxy": True, "indexes": []}
+            changes, "testapp", 0, 0, name="AuthorProxy", options={"proxy": True, "indexes": [], "constraints": []}
         )
         # Now, we test turning a proxy model into a non-proxy model
         # It should delete the proxy then make the real one
diff --git a/tests/migrations/test_base.py b/tests/migrations/test_base.py
index 84a5117751..7fcbaffd24 100644
--- a/tests/migrations/test_base.py
+++ b/tests/migrations/test_base.py
@@ -67,6 +67,17 @@ class MigrationTestBase(TransactionTestCase):
     def assertIndexNotExists(self, table, columns):
         return self.assertIndexExists(table, columns, False)
 
+    def assertConstraintExists(self, table, name, value=True, using='default'):
+        with connections[using].cursor() as cursor:
+            constraints = connections[using].introspection.get_constraints(cursor, table).items()
+            self.assertEqual(
+                value,
+                any(c['check'] for n, c in constraints if n == name),
+            )
+
+    def assertConstraintNotExists(self, table, name):
+        return self.assertConstraintExists(table, name, False)
+
     def assertFKExists(self, table, columns, to, value=True, using='default'):
         with connections[using].cursor() as cursor:
             self.assertEqual(
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index d70feaacdb..b1581042f7 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -53,7 +53,7 @@ class OperationTestBase(MigrationTestBase):
     def set_up_test_model(
             self, app_label, second_model=False, third_model=False, index=False, multicol_index=False,
             related_model=False, mti_model=False, proxy_model=False, manager_model=False,
-            unique_together=False, options=False, db_table=None, index_together=False):
+            unique_together=False, options=False, db_table=None, index_together=False, check_constraint=False):
         """
         Creates a test model state and database table.
         """
@@ -106,6 +106,11 @@ class OperationTestBase(MigrationTestBase):
                 "Pony",
                 models.Index(fields=["pink", "weight"], name="pony_test_idx")
             ))
+        if check_constraint:
+            operations.append(migrations.AddConstraint(
+                "Pony",
+                models.CheckConstraint(models.Q(pink__gt=2), name="pony_test_constraint")
+            ))
         if second_model:
             operations.append(migrations.CreateModel(
                 "Stable",
@@ -462,6 +467,45 @@ class OperationTests(OperationTestBase):
         self.assertTableNotExists("test_crummo_unmanagedpony")
         self.assertTableExists("test_crummo_pony")
 
+    @skipUnlessDBFeature('supports_table_check_constraints')
+    def test_create_model_with_constraint(self):
+        where = models.Q(pink__gt=2)
+        check_constraint = models.CheckConstraint(where, name='test_constraint_pony_pink_gt_2')
+        operation = migrations.CreateModel(
+            "Pony",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("pink", models.IntegerField(default=3)),
+            ],
+            options={'constraints': [check_constraint]},
+        )
+
+        # Test the state alteration
+        project_state = ProjectState()
+        new_state = project_state.clone()
+        operation.state_forwards("test_crmo", new_state)
+        self.assertEqual(len(new_state.models['test_crmo', 'pony'].options['constraints']), 1)
+
+        # Test database alteration
+        self.assertTableNotExists("test_crmo_pony")
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_crmo", editor, project_state, new_state)
+        self.assertTableExists("test_crmo_pony")
+        with connection.cursor() as cursor:
+            with self.assertRaises(IntegrityError):
+                cursor.execute("INSERT INTO test_crmo_pony (id, pink) VALUES (1, 1)")
+
+        # Test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_crmo", editor, new_state, project_state)
+        self.assertTableNotExists("test_crmo_pony")
+
+        # Test deconstruction
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "CreateModel")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(definition[2]['options']['constraints'], [check_constraint])
+
     def test_create_model_managers(self):
         """
         The managers on a model are set.
@@ -1708,6 +1752,87 @@ class OperationTests(OperationTestBase):
         operation = migrations.AlterIndexTogether("Pony", None)
         self.assertEqual(operation.describe(), "Alter index_together for Pony (0 constraint(s))")
 
+    @skipUnlessDBFeature('supports_table_check_constraints')
+    def test_add_constraint(self):
+        """Test the AddConstraint operation."""
+        project_state = self.set_up_test_model('test_addconstraint')
+
+        where = models.Q(pink__gt=2)
+        check_constraint = models.CheckConstraint(where, name='test_constraint_pony_pink_gt_2')
+        operation = migrations.AddConstraint('Pony', check_constraint)
+        self.assertEqual(operation.describe(), 'Create constraint test_constraint_pony_pink_gt_2 on model Pony')
+
+        new_state = project_state.clone()
+        operation.state_forwards('test_addconstraint', new_state)
+        self.assertEqual(len(new_state.models['test_addconstraint', 'pony'].options['constraints']), 1)
+
+        # Test database alteration
+        with connection.cursor() as cursor:
+            with atomic():
+                cursor.execute("INSERT INTO test_addconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+            cursor.execute("DELETE FROM test_addconstraint_pony")
+
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_addconstraint", editor, project_state, new_state)
+
+        with connection.cursor() as cursor:
+            with self.assertRaises(IntegrityError):
+                cursor.execute("INSERT INTO test_addconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+
+        # Test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_addconstraint", editor, new_state, project_state)
+
+        with connection.cursor() as cursor:
+            with atomic():
+                cursor.execute("INSERT INTO test_addconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+            cursor.execute("DELETE FROM test_addconstraint_pony")
+
+        # Test deconstruction
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "AddConstraint")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(definition[2], {'model_name': "Pony", 'constraint': check_constraint})
+
+    @skipUnlessDBFeature('supports_table_check_constraints')
+    def test_remove_constraint(self):
+        """Test the RemoveConstraint operation."""
+        project_state = self.set_up_test_model("test_removeconstraint", check_constraint=True)
+        self.assertTableExists("test_removeconstraint_pony")
+        operation = migrations.RemoveConstraint("Pony", "pony_test_constraint")
+        self.assertEqual(operation.describe(), "Remove constraint pony_test_constraint from model Pony")
+        new_state = project_state.clone()
+        operation.state_forwards("test_removeconstraint", new_state)
+        # Test state alteration
+        self.assertEqual(len(new_state.models["test_removeconstraint", "pony"].options['constraints']), 0)
+
+        with connection.cursor() as cursor:
+            with self.assertRaises(IntegrityError):
+                cursor.execute("INSERT INTO test_removeconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+
+        # Test database alteration
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_removeconstraint", editor, project_state, new_state)
+
+        with connection.cursor() as cursor:
+            with atomic():
+                cursor.execute("INSERT INTO test_removeconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+            cursor.execute("DELETE FROM test_removeconstraint_pony")
+
+        # Test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_removeconstraint", editor, new_state, project_state)
+
+        with connection.cursor() as cursor:
+            with self.assertRaises(IntegrityError):
+                cursor.execute("INSERT INTO test_removeconstraint_pony (id, pink, weight) VALUES (1, 1, 1.0)")
+
+        # Test deconstruction
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "RemoveConstraint")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(definition[2], {'model_name': "Pony", 'name': "pony_test_constraint"})
+
     def test_alter_model_options(self):
         """
         Tests the AlterModelOptions operation.
diff --git a/tests/migrations/test_state.py b/tests/migrations/test_state.py
index 255e14beff..6a7a087ac5 100644
--- a/tests/migrations/test_state.py
+++ b/tests/migrations/test_state.py
@@ -127,7 +127,12 @@ class StateTests(SimpleTestCase):
         self.assertIs(author_state.fields[3][1].null, True)
         self.assertEqual(
             author_state.options,
-            {"unique_together": {("name", "bio")}, "index_together": {("bio", "age")}, "indexes": []}
+            {
+                "unique_together": {("name", "bio")},
+                "index_together": {("bio", "age")},
+                "indexes": [],
+                "constraints": [],
+            }
         )
         self.assertEqual(author_state.bases, (models.Model,))
 
@@ -139,14 +144,17 @@ class StateTests(SimpleTestCase):
         self.assertEqual(book_state.fields[3][1].__class__.__name__, "ManyToManyField")
         self.assertEqual(
             book_state.options,
-            {"verbose_name": "tome", "db_table": "test_tome", "indexes": [book_index]},
+            {"verbose_name": "tome", "db_table": "test_tome", "indexes": [book_index], "constraints": []},
         )
         self.assertEqual(book_state.bases, (models.Model,))
 
         self.assertEqual(author_proxy_state.app_label, "migrations")
         self.assertEqual(author_proxy_state.name, "AuthorProxy")
         self.assertEqual(author_proxy_state.fields, [])
-        self.assertEqual(author_proxy_state.options, {"proxy": True, "ordering": ["name"], "indexes": []})
+        self.assertEqual(
+            author_proxy_state.options,
+            {"proxy": True, "ordering": ["name"], "indexes": [], "constraints": []},
+        )
         self.assertEqual(author_proxy_state.bases, ("migrations.author",))
 
         self.assertEqual(sub_author_state.app_label, "migrations")
@@ -1002,7 +1010,7 @@ class ModelStateTests(SimpleTestCase):
         self.assertEqual(author_state.fields[1][1].max_length, 255)
         self.assertIs(author_state.fields[2][1].null, False)
         self.assertIs(author_state.fields[3][1].null, True)
-        self.assertEqual(author_state.options, {'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': []})
+        self.assertEqual(author_state.options, {'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': [], "constraints": []})
         self.assertEqual(author_state.bases, (models.Model,))
         self.assertEqual(author_state.managers, [])
 
@@ -1047,7 +1055,7 @@ class ModelStateTests(SimpleTestCase):
         self.assertEqual(station_state.fields[2][1].null, False)
         self.assertEqual(
             station_state.options,
-            {'abstract': False, 'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': []}
+            {'abstract': False, 'swappable': 'TEST_SWAPPABLE_MODEL', 'indexes': [], 'constraints': []}
         )
         self.assertEqual(station_state.bases, ('migrations.searchablelocation',))
         self.assertEqual(station_state.managers, [])
@@ -1129,6 +1137,21 @@ class ModelStateTests(SimpleTestCase):
         index_names = [index.name for index in model_state.options['indexes']]
         self.assertEqual(index_names, ['foo_idx'])
 
+    @isolate_apps('migrations')
+    def test_from_model_constraints(self):
+        class ModelWithConstraints(models.Model):
+            size = models.IntegerField()
+
+            class Meta:
+                constraints = [models.CheckConstraint(models.Q(size__gt=1), 'size_gt_1')]
+
+        state = ModelState.from_model(ModelWithConstraints)
+        model_constraints = ModelWithConstraints._meta.constraints
+        state_constraints = state.options['constraints']
+        self.assertEqual(model_constraints, state_constraints)
+        self.assertIsNot(model_constraints, state_constraints)
+        self.assertIsNot(model_constraints[0], state_constraints[0])
+
 
 class RelatedModelsTests(SimpleTestCase):
 
diff --git a/tests/queries/test_query.py b/tests/queries/test_query.py
new file mode 100644
index 0000000000..10ea8eb0f2
--- /dev/null
+++ b/tests/queries/test_query.py
@@ -0,0 +1,95 @@
+from datetime import datetime
+
+from django.core.exceptions import FieldError
+from django.db.models import CharField, F, Q
+from django.db.models.expressions import SimpleCol
+from django.db.models.fields.related_lookups import RelatedIsNull
+from django.db.models.functions import Lower
+from django.db.models.lookups import Exact, GreaterThan, IsNull, LessThan
+from django.db.models.sql.query import Query
+from django.db.models.sql.where import OR
+from django.test import TestCase
+
+from .models import Author, Item, ObjectC, Ranking
+
+
+class TestQuery(TestCase):
+    def test_simple_query(self):
+        query = Query(Author)
+        where = query.build_where(Q(num__gt=2))
+        lookup = where.children[0]
+        self.assertIsInstance(lookup, GreaterThan)
+        self.assertEqual(lookup.rhs, 2)
+        self.assertEqual(lookup.lhs.target, Author._meta.get_field('num'))
+
+    def test_complex_query(self):
+        query = Query(Author)
+        where = query.build_where(Q(num__gt=2) | Q(num__lt=0))
+        self.assertEqual(where.connector, OR)
+
+        lookup = where.children[0]
+        self.assertIsInstance(lookup, GreaterThan)
+        self.assertEqual(lookup.rhs, 2)
+        self.assertEqual(lookup.lhs.target, Author._meta.get_field('num'))
+
+        lookup = where.children[1]
+        self.assertIsInstance(lookup, LessThan)
+        self.assertEqual(lookup.rhs, 0)
+        self.assertEqual(lookup.lhs.target, Author._meta.get_field('num'))
+
+    def test_multiple_fields(self):
+        query = Query(Item)
+        where = query.build_where(Q(modified__gt=F('created')))
+        lookup = where.children[0]
+        self.assertIsInstance(lookup, GreaterThan)
+        self.assertIsInstance(lookup.rhs, SimpleCol)
+        self.assertIsInstance(lookup.lhs, SimpleCol)
+        self.assertEqual(lookup.rhs.target, Item._meta.get_field('created'))
+        self.assertEqual(lookup.lhs.target, Item._meta.get_field('modified'))
+
+    def test_transform(self):
+        query = Query(Author)
+        CharField.register_lookup(Lower, 'lower')
+        try:
+            where = query.build_where(~Q(name__lower='foo'))
+        finally:
+            CharField._unregister_lookup(Lower, 'lower')
+        lookup = where.children[0]
+        self.assertIsInstance(lookup, Exact)
+        self.assertIsInstance(lookup.lhs, Lower)
+        self.assertIsInstance(lookup.lhs.lhs, SimpleCol)
+        self.assertEqual(lookup.lhs.lhs.target, Author._meta.get_field('name'))
+
+    def test_negated_nullable(self):
+        query = Query(Item)
+        where = query.build_where(~Q(modified__lt=datetime(2017, 1, 1)))
+        self.assertTrue(where.negated)
+        lookup = where.children[0]
+        self.assertIsInstance(lookup, LessThan)
+        self.assertEqual(lookup.lhs.target, Item._meta.get_field('modified'))
+        lookup = where.children[1]
+        self.assertIsInstance(lookup, IsNull)
+        self.assertEqual(lookup.lhs.target, Item._meta.get_field('modified'))
+
+    def test_foreign_key(self):
+        query = Query(Item)
+        msg = 'Joined field references are not permitted in this query'
+        with self.assertRaisesMessage(FieldError, msg):
+            query.build_where(Q(creator__num__gt=2))
+
+    def test_foreign_key_f(self):
+        query = Query(Ranking)
+        with self.assertRaises(FieldError):
+            query.build_where(Q(rank__gt=F('author__num')))
+
+    def test_foreign_key_exclusive(self):
+        query = Query(ObjectC)
+        where = query.build_where(Q(objecta=None) | Q(objectb=None))
+        a_isnull = where.children[0]
+        self.assertIsInstance(a_isnull, RelatedIsNull)
+        self.assertIsInstance(a_isnull.lhs, SimpleCol)
+        self.assertEqual(a_isnull.lhs.target, ObjectC._meta.get_field('objecta'))
+        b_isnull = where.children[1]
+        self.assertIsInstance(b_isnull, RelatedIsNull)
+        self.assertIsInstance(b_isnull.lhs, SimpleCol)
+        self.assertEqual(b_isnull.lhs.target, ObjectC._meta.get_field('objectb'))