Fixed #20581 -- Added support for deferrable unique constraints.

14 files changed, 457 insertions, 16 deletions

diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index c2b92ecdee..a8f55f966c 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -20,6 +20,8 @@ class BaseDatabaseFeatures:
     # Does the backend allow inserting duplicate rows when a unique_together
     # constraint exists and some fields are nullable but not all of them?
     supports_partially_nullable_unique_constraints = True
+    # Does the backend support initially deferrable unique constraints?
+    supports_deferrable_unique_constraints = False
 
     can_use_chunked_reads = True
     can_return_columns_from_insert = False
diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 104f569a07..2b2ad9cdb4 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -5,7 +5,7 @@ from django.db.backends.ddl_references import (
     Columns, ForeignKeyName, IndexName, Statement, Table,
 )
 from django.db.backends.utils import names_digest, split_identifier
-from django.db.models import Index
+from django.db.models import Deferrable, Index
 from django.db.transaction import TransactionManagementError, atomic
 from django.utils import timezone
 
@@ -65,7 +65,7 @@ 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_unique_constraint = "UNIQUE (%(columns)s)"
+    sql_unique_constraint = "UNIQUE (%(columns)s)%(deferrable)s"
     sql_check_constraint = "CHECK (%(check)s)"
     sql_delete_constraint = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
     sql_constraint = "CONSTRAINT %(name)s %(constraint)s"
@@ -73,7 +73,7 @@ class BaseDatabaseSchemaEditor:
     sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
     sql_delete_check = sql_delete_constraint
 
-    sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
+    sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)%(deferrable)s"
     sql_delete_unique = sql_delete_constraint
 
     sql_create_fk = (
@@ -1075,7 +1075,20 @@ class BaseDatabaseSchemaEditor:
     def _delete_fk_sql(self, model, name):
         return self._delete_constraint_sql(self.sql_delete_fk, model, name)
 
-    def _unique_sql(self, model, fields, name, condition=None):
+    def _deferrable_constraint_sql(self, deferrable):
+        if deferrable is None:
+            return ''
+        if deferrable == Deferrable.DEFERRED:
+            return ' DEFERRABLE INITIALLY DEFERRED'
+        if deferrable == Deferrable.IMMEDIATE:
+            return ' DEFERRABLE INITIALLY IMMEDIATE'
+
+    def _unique_sql(self, model, fields, name, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
         if condition:
             # Databases support conditional unique constraints via a unique
             # index.
@@ -1085,13 +1098,20 @@ class BaseDatabaseSchemaEditor:
             return None
         constraint = self.sql_unique_constraint % {
             'columns': ', '.join(map(self.quote_name, fields)),
+            'deferrable': self._deferrable_constraint_sql(deferrable),
         }
         return self.sql_constraint % {
             'name': self.quote_name(name),
             'constraint': constraint,
         }
 
-    def _create_unique_sql(self, model, columns, name=None, condition=None):
+    def _create_unique_sql(self, model, columns, name=None, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
+
         def create_unique_name(*args, **kwargs):
             return self.quote_name(self._create_index_name(*args, **kwargs))
 
@@ -1113,9 +1133,15 @@ class BaseDatabaseSchemaEditor:
             name=name,
             columns=columns,
             condition=self._index_condition_sql(condition),
+            deferrable=self._deferrable_constraint_sql(deferrable),
         )
 
-    def _delete_unique_sql(self, model, name, condition=None):
+    def _delete_unique_sql(self, model, name, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
         if condition:
             return (
                 self._delete_constraint_sql(self.sql_delete_index, model, name)
diff --git a/django/db/backends/oracle/base.py b/django/db/backends/oracle/base.py
index 8285aacdc1..e9ec2bac51 100644
--- a/django/db/backends/oracle/base.py
+++ b/django/db/backends/oracle/base.py
@@ -71,9 +71,17 @@ def wrap_oracle_errors():
         #  message = 'ORA-02091: transaction rolled back
         #            'ORA-02291: integrity constraint (TEST_DJANGOTEST.SYS
         #               _C00102056) violated - parent key not found'
+        #            or:
+        #            'ORA-00001: unique constraint (DJANGOTEST.DEFERRABLE_
+        #               PINK_CONSTRAINT) violated
         # Convert that case to Django's IntegrityError exception.
         x = e.args[0]
-        if hasattr(x, 'code') and hasattr(x, 'message') and x.code == 2091 and 'ORA-02291' in x.message:
+        if (
+            hasattr(x, 'code') and
+            hasattr(x, 'message') and
+            x.code == 2091 and
+            ('ORA-02291' in x.message or 'ORA-00001' in x.message)
+        ):
             raise IntegrityError(*tuple(e.args))
         raise
 
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index 24b281281d..3782874512 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -17,6 +17,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_native_duration_field = True
     can_defer_constraint_checks = True
     supports_partially_nullable_unique_constraints = False
+    supports_deferrable_unique_constraints = True
     truncates_names = True
     supports_tablespaces = True
     supports_sequence_reset = False
diff --git a/django/db/backends/postgresql/features.py b/django/db/backends/postgresql/features.py
index 533c8dc282..3b4199fa78 100644
--- a/django/db/backends/postgresql/features.py
+++ b/django/db/backends/postgresql/features.py
@@ -56,6 +56,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_aggregate_filter_clause = True
     supported_explain_formats = {'JSON', 'TEXT', 'XML', 'YAML'}
     validates_explain_options = False  # A query will error on invalid options.
+    supports_deferrable_unique_constraints = True
 
     @cached_property
     def is_postgresql_9_6(self):
diff --git a/django/db/models/base.py b/django/db/models/base.py
index 0f8a8c50f3..6c9e9d3707 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -1904,6 +1904,25 @@ class Model(metaclass=ModelBase):
                         id='models.W036',
                     )
                 )
+            if not (
+                connection.features.supports_deferrable_unique_constraints or
+                'supports_deferrable_unique_constraints' in cls._meta.required_db_features
+            ) and any(
+                isinstance(constraint, UniqueConstraint) and constraint.deferrable is not None
+                for constraint in cls._meta.constraints
+            ):
+                errors.append(
+                    checks.Warning(
+                        '%s does not support deferrable unique 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.W038',
+                    )
+                )
         return errors
 
 
diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 98912a6467..64bd60484e 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -1,7 +1,9 @@
+from enum import Enum
+
 from django.db.models.query_utils import Q
 from django.db.models.sql.query import Query
 
-__all__ = ['CheckConstraint', 'UniqueConstraint']
+__all__ = ['CheckConstraint', 'Deferrable', 'UniqueConstraint']
 
 
 class BaseConstraint:
@@ -69,14 +71,28 @@ class CheckConstraint(BaseConstraint):
         return path, args, kwargs
 
 
+class Deferrable(Enum):
+    DEFERRED = 'deferred'
+    IMMEDIATE = 'immediate'
+
+
 class UniqueConstraint(BaseConstraint):
-    def __init__(self, *, fields, name, condition=None):
+    def __init__(self, *, fields, name, condition=None, deferrable=None):
         if not fields:
             raise ValueError('At least one field is required to define a unique constraint.')
         if not isinstance(condition, (type(None), Q)):
             raise ValueError('UniqueConstraint.condition must be a Q instance.')
+        if condition and deferrable:
+            raise ValueError(
+                'UniqueConstraint with conditions cannot be deferred.'
+            )
+        if not isinstance(deferrable, (type(None), Deferrable)):
+            raise ValueError(
+                'UniqueConstraint.deferrable must be a Deferrable instance.'
+            )
         self.fields = tuple(fields)
         self.condition = condition
+        self.deferrable = deferrable
         super().__init__(name)
 
     def _get_condition_sql(self, model, schema_editor):
@@ -91,21 +107,30 @@ class UniqueConstraint(BaseConstraint):
     def constraint_sql(self, model, schema_editor):
         fields = [model._meta.get_field(field_name).column for field_name in self.fields]
         condition = self._get_condition_sql(model, schema_editor)
-        return schema_editor._unique_sql(model, fields, self.name, condition=condition)
+        return schema_editor._unique_sql(
+            model, fields, self.name, condition=condition,
+            deferrable=self.deferrable,
+        )
 
     def create_sql(self, model, schema_editor):
         fields = [model._meta.get_field(field_name).column for field_name in self.fields]
         condition = self._get_condition_sql(model, schema_editor)
-        return schema_editor._create_unique_sql(model, fields, self.name, condition=condition)
+        return schema_editor._create_unique_sql(
+            model, fields, self.name, condition=condition,
+            deferrable=self.deferrable,
+        )
 
     def remove_sql(self, model, schema_editor):
         condition = self._get_condition_sql(model, schema_editor)
-        return schema_editor._delete_unique_sql(model, self.name, condition=condition)
+        return schema_editor._delete_unique_sql(
+            model, self.name, condition=condition, deferrable=self.deferrable,
+        )
 
     def __repr__(self):
-        return '<%s: fields=%r name=%r%s>' % (
+        return '<%s: fields=%r name=%r%s%s>' % (
             self.__class__.__name__, self.fields, self.name,
             '' if self.condition is None else ' condition=%s' % self.condition,
+            '' if self.deferrable is None else ' deferrable=%s' % self.deferrable,
         )
 
     def __eq__(self, other):
@@ -113,7 +138,8 @@ class UniqueConstraint(BaseConstraint):
             return (
                 self.name == other.name and
                 self.fields == other.fields and
-                self.condition == other.condition
+                self.condition == other.condition and
+                self.deferrable == other.deferrable
             )
         return super().__eq__(other)
 
@@ -122,4 +148,6 @@ class UniqueConstraint(BaseConstraint):
         kwargs['fields'] = self.fields
         if self.condition:
             kwargs['condition'] = self.condition
+        if self.deferrable:
+            kwargs['deferrable'] = self.deferrable
         return path, args, kwargs
diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index d9b56c3c58..daf651392f 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -354,6 +354,8 @@ Models
 * **models.W036**: ``<database>`` does not support unique constraints with
   conditions.
 * **models.W037**: ``<database>`` does not support indexes with conditions.
+* **models.W038**: ``<database>`` does not support deferrable unique
+  constraints.
 
 Security
 --------
diff --git a/docs/ref/models/constraints.txt b/docs/ref/models/constraints.txt
index 9e3119f600..00e907a882 100644
--- a/docs/ref/models/constraints.txt
+++ b/docs/ref/models/constraints.txt
@@ -76,7 +76,7 @@ The name of the constraint.
 ``UniqueConstraint``
 ====================
 
-.. class:: UniqueConstraint(*, fields, name, condition=None)
+.. class:: UniqueConstraint(*, fields, name, condition=None, deferrable=None)
 
     Creates a unique constraint in the database.
 
@@ -119,3 +119,35 @@ ensures that each user only has one draft.
 
 These conditions have the same database restrictions as
 :attr:`Index.condition`.
+
+``deferrable``
+--------------
+
+.. attribute:: UniqueConstraint.deferrable
+
+.. versionadded:: 3.1
+
+Set this parameter to create a deferrable unique constraint. Accepted values
+are ``Deferrable.DEFERRED`` or ``Deferrable.IMMEDIATE``. For example::
+
+    from django.db.models import Deferrable, UniqueConstraint
+
+    UniqueConstraint(
+        name='unique_order',
+        fields=['order'],
+        deferrable=Deferrable.DEFERRED,
+    )
+
+By default constraints are not deferred. A deferred constraint will not be
+enforced until the end of the transaction. An immediate constraint will be
+enforced immediately after every command.
+
+.. admonition:: MySQL, MariaDB, and SQLite.
+
+    Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as
+    neither supports them.
+
+.. warning::
+
+    Deferred unique constraints may lead to a `performance penalty
+    <https://www.postgresql.org/docs/current/sql-createtable.html#id-1.9.3.85.9.4>`_.
diff --git a/docs/releases/3.1.txt b/docs/releases/3.1.txt
index 1de4f24684..ae482f0129 100644
--- a/docs/releases/3.1.txt
+++ b/docs/releases/3.1.txt
@@ -381,6 +381,9 @@ Models
   <sqlite3.Connection.create_function>` on Python 3.8+. This allows using them
   in check constraints and partial indexes.
 
+* The new :attr:`.UniqueConstraint.deferrable` attribute allows creating
+  deferrable unique constraints.
+
 Pagination
 ~~~~~~~~~~
 
diff --git a/tests/constraints/models.py b/tests/constraints/models.py
index 98955498d4..3d091f6ccf 100644
--- a/tests/constraints/models.py
+++ b/tests/constraints/models.py
@@ -59,6 +59,28 @@ class UniqueConstraintConditionProduct(models.Model):
         ]
 
 
+class UniqueConstraintDeferrable(models.Model):
+    name = models.CharField(max_length=255)
+    shelf = models.CharField(max_length=31)
+
+    class Meta:
+        required_db_features = {
+            'supports_deferrable_unique_constraints',
+        }
+        constraints = [
+            models.UniqueConstraint(
+                fields=['name'],
+                name='name_init_deferred_uniq',
+                deferrable=models.Deferrable.DEFERRED,
+            ),
+            models.UniqueConstraint(
+                fields=['shelf'],
+                name='sheld_init_immediate_uniq',
+                deferrable=models.Deferrable.IMMEDIATE,
+            ),
+        ]
+
+
 class AbstractModel(models.Model):
     age = models.IntegerField()
 
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index 85edb51aa7..8eb62a940d 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -3,11 +3,12 @@ from unittest import mock
 from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection, models
 from django.db.models.constraints import BaseConstraint
+from django.db.transaction import atomic
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 
 from .models import (
     ChildModel, Product, UniqueConstraintConditionProduct,
-    UniqueConstraintProduct,
+    UniqueConstraintDeferrable, UniqueConstraintProduct,
 )
 
 
@@ -166,6 +167,20 @@ class UniqueConstraintTests(TestCase):
             ),
         )
 
+    def test_eq_with_deferrable(self):
+        constraint_1 = models.UniqueConstraint(
+            fields=['foo', 'bar'],
+            name='unique',
+            deferrable=models.Deferrable.DEFERRED,
+        )
+        constraint_2 = models.UniqueConstraint(
+            fields=['foo', 'bar'],
+            name='unique',
+            deferrable=models.Deferrable.IMMEDIATE,
+        )
+        self.assertEqual(constraint_1, constraint_1)
+        self.assertNotEqual(constraint_1, constraint_2)
+
     def test_repr(self):
         fields = ['foo', 'bar']
         name = 'unique_fields'
@@ -187,6 +202,18 @@ class UniqueConstraintTests(TestCase):
             "condition=(AND: ('foo', F(bar)))>",
         )
 
+    def test_repr_with_deferrable(self):
+        constraint = models.UniqueConstraint(
+            fields=['foo', 'bar'],
+            name='unique_fields',
+            deferrable=models.Deferrable.IMMEDIATE,
+        )
+        self.assertEqual(
+            repr(constraint),
+            "<UniqueConstraint: fields=('foo', 'bar') name='unique_fields' "
+            "deferrable=Deferrable.IMMEDIATE>",
+        )
+
     def test_deconstruction(self):
         fields = ['foo', 'bar']
         name = 'unique_fields'
@@ -206,6 +233,23 @@ class UniqueConstraintTests(TestCase):
         self.assertEqual(args, ())
         self.assertEqual(kwargs, {'fields': tuple(fields), 'name': name, 'condition': condition})
 
+    def test_deconstruction_with_deferrable(self):
+        fields = ['foo']
+        name = 'unique_fields'
+        constraint = models.UniqueConstraint(
+            fields=fields,
+            name=name,
+            deferrable=models.Deferrable.DEFERRED,
+        )
+        path, args, kwargs = constraint.deconstruct()
+        self.assertEqual(path, 'django.db.models.UniqueConstraint')
+        self.assertEqual(args, ())
+        self.assertEqual(kwargs, {
+            'fields': tuple(fields),
+            'name': name,
+            'deferrable': models.Deferrable.DEFERRED,
+        })
+
     def test_database_constraint(self):
         with self.assertRaises(IntegrityError):
             UniqueConstraintProduct.objects.create(name=self.p1.name, color=self.p1.color)
@@ -238,3 +282,54 @@ class UniqueConstraintTests(TestCase):
     def test_condition_must_be_q(self):
         with self.assertRaisesMessage(ValueError, 'UniqueConstraint.condition must be a Q instance.'):
             models.UniqueConstraint(name='uniq', fields=['name'], condition='invalid')
+
+    @skipUnlessDBFeature('supports_deferrable_unique_constraints')
+    def test_initially_deferred_database_constraint(self):
+        obj_1 = UniqueConstraintDeferrable.objects.create(name='p1', shelf='front')
+        obj_2 = UniqueConstraintDeferrable.objects.create(name='p2', shelf='back')
+
+        def swap():
+            obj_1.name, obj_2.name = obj_2.name, obj_1.name
+            obj_1.save()
+            obj_2.save()
+
+        swap()
+        # Behavior can be changed with SET CONSTRAINTS.
+        with self.assertRaises(IntegrityError):
+            with atomic(), connection.cursor() as cursor:
+                constraint_name = connection.ops.quote_name('name_init_deferred_uniq')
+                cursor.execute('SET CONSTRAINTS %s IMMEDIATE' % constraint_name)
+                swap()
+
+    @skipUnlessDBFeature('supports_deferrable_unique_constraints')
+    def test_initially_immediate_database_constraint(self):
+        obj_1 = UniqueConstraintDeferrable.objects.create(name='p1', shelf='front')
+        obj_2 = UniqueConstraintDeferrable.objects.create(name='p2', shelf='back')
+        obj_1.shelf, obj_2.shelf = obj_2.shelf, obj_1.shelf
+        with self.assertRaises(IntegrityError), atomic():
+            obj_1.save()
+        # Behavior can be changed with SET CONSTRAINTS.
+        with connection.cursor() as cursor:
+            constraint_name = connection.ops.quote_name('sheld_init_immediate_uniq')
+            cursor.execute('SET CONSTRAINTS %s DEFERRED' % constraint_name)
+            obj_1.save()
+            obj_2.save()
+
+    def test_deferrable_with_condition(self):
+        message = 'UniqueConstraint with conditions cannot be deferred.'
+        with self.assertRaisesMessage(ValueError, message):
+            models.UniqueConstraint(
+                fields=['name'],
+                name='name_without_color_unique',
+                condition=models.Q(color__isnull=True),
+                deferrable=models.Deferrable.DEFERRED,
+            )
+
+    def test_invalid_defer_argument(self):
+        message = 'UniqueConstraint.deferrable must be a Deferrable instance.'
+        with self.assertRaisesMessage(ValueError, message):
+            models.UniqueConstraint(
+                fields=['name'],
+                name='name_invalid',
+                deferrable='invalid',
+            )
diff --git a/tests/invalid_models_tests/test_models.py b/tests/invalid_models_tests/test_models.py
index f3f0bc8bd5..6bfdf2e736 100644
--- a/tests/invalid_models_tests/test_models.py
+++ b/tests/invalid_models_tests/test_models.py
@@ -1414,3 +1414,47 @@ class ConstraintsTests(TestCase):
                 ]
 
         self.assertEqual(Model.check(databases=self.databases), [])
+
+    def test_deferrable_unique_constraint(self):
+        class Model(models.Model):
+            age = models.IntegerField()
+
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(
+                        fields=['age'],
+                        name='unique_age_deferrable',
+                        deferrable=models.Deferrable.DEFERRED,
+                    ),
+                ]
+
+        errors = Model.check(databases=self.databases)
+        expected = [] if connection.features.supports_deferrable_unique_constraints else [
+            Warning(
+                '%s does not support deferrable unique 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.W038',
+            ),
+        ]
+        self.assertEqual(errors, expected)
+
+    def test_deferrable_unique_constraint_required_db_features(self):
+        class Model(models.Model):
+            age = models.IntegerField()
+
+            class Meta:
+                required_db_features = {'supports_deferrable_unique_constraints'}
+                constraints = [
+                    models.UniqueConstraint(
+                        fields=['age'],
+                        name='unique_age_deferrable',
+                        deferrable=models.Deferrable.IMMEDIATE,
+                    ),
+                ]
+
+        self.assertEqual(Model.check(databases=self.databases), [])
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index 401cae6eae..e985535679 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -393,6 +393,60 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(definition[2]['options']['constraints'], [partial_unique_constraint])
 
+    def test_create_model_with_deferred_unique_constraint(self):
+        deferred_unique_constraint = models.UniqueConstraint(
+            fields=['pink'],
+            name='deferrable_pink_constraint',
+            deferrable=models.Deferrable.DEFERRED,
+        )
+        operation = migrations.CreateModel(
+            'Pony',
+            [
+                ('id', models.AutoField(primary_key=True)),
+                ('pink', models.IntegerField(default=3)),
+            ],
+            options={'constraints': [deferred_unique_constraint]},
+        )
+        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)
+        self.assertTableNotExists('test_crmo_pony')
+        # Create table.
+        with connection.schema_editor() as editor:
+            operation.database_forwards('test_crmo', editor, project_state, new_state)
+        self.assertTableExists('test_crmo_pony')
+        Pony = new_state.apps.get_model('test_crmo', 'Pony')
+        Pony.objects.create(pink=1)
+        if connection.features.supports_deferrable_unique_constraints:
+            # Unique constraint is deferred.
+            with transaction.atomic():
+                obj = Pony.objects.create(pink=1)
+                obj.pink = 2
+                obj.save()
+            # Constraint behavior can be changed with SET CONSTRAINTS.
+            with self.assertRaises(IntegrityError):
+                with transaction.atomic(), connection.cursor() as cursor:
+                    quoted_name = connection.ops.quote_name(deferred_unique_constraint.name)
+                    cursor.execute('SET CONSTRAINTS %s IMMEDIATE' % quoted_name)
+                    obj = Pony.objects.create(pink=1)
+                    obj.pink = 3
+                    obj.save()
+        else:
+            Pony.objects.create(pink=1)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards('test_crmo', editor, new_state, project_state)
+        self.assertTableNotExists('test_crmo_pony')
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], 'CreateModel')
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2]['options']['constraints'],
+            [deferred_unique_constraint],
+        )
+
     def test_create_model_managers(self):
         """
         The managers on a model are set.
@@ -2046,6 +2100,110 @@ class OperationTests(OperationTestBase):
             'name': 'test_constraint_pony_pink_for_weight_gt_5_uniq',
         })
 
+    def test_add_deferred_unique_constraint(self):
+        app_label = 'test_adddeferred_uc'
+        project_state = self.set_up_test_model(app_label)
+        deferred_unique_constraint = models.UniqueConstraint(
+            fields=['pink'],
+            name='deferred_pink_constraint_add',
+            deferrable=models.Deferrable.DEFERRED,
+        )
+        operation = migrations.AddConstraint('Pony', deferred_unique_constraint)
+        self.assertEqual(
+            operation.describe(),
+            'Create constraint deferred_pink_constraint_add on model Pony',
+        )
+        # Add constraint.
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, 'pony'].options['constraints']), 1)
+        Pony = new_state.apps.get_model(app_label, 'Pony')
+        self.assertEqual(len(Pony._meta.constraints), 1)
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        Pony.objects.create(pink=1, weight=4.0)
+        if connection.features.supports_deferrable_unique_constraints:
+            # Unique constraint is deferred.
+            with transaction.atomic():
+                obj = Pony.objects.create(pink=1, weight=4.0)
+                obj.pink = 2
+                obj.save()
+            # Constraint behavior can be changed with SET CONSTRAINTS.
+            with self.assertRaises(IntegrityError):
+                with transaction.atomic(), connection.cursor() as cursor:
+                    quoted_name = connection.ops.quote_name(deferred_unique_constraint.name)
+                    cursor.execute('SET CONSTRAINTS %s IMMEDIATE' % quoted_name)
+                    obj = Pony.objects.create(pink=1, weight=4.0)
+                    obj.pink = 3
+                    obj.save()
+        else:
+            Pony.objects.create(pink=1, weight=4.0)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        # Constraint doesn't work.
+        Pony.objects.create(pink=1, weight=4.0)
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], 'AddConstraint')
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2],
+            {'model_name': 'Pony', 'constraint': deferred_unique_constraint},
+        )
+
+    def test_remove_deferred_unique_constraint(self):
+        app_label = 'test_removedeferred_uc'
+        deferred_unique_constraint = models.UniqueConstraint(
+            fields=['pink'],
+            name='deferred_pink_constraint_rm',
+            deferrable=models.Deferrable.DEFERRED,
+        )
+        project_state = self.set_up_test_model(app_label, constraints=[deferred_unique_constraint])
+        operation = migrations.RemoveConstraint('Pony', deferred_unique_constraint.name)
+        self.assertEqual(
+            operation.describe(),
+            'Remove constraint deferred_pink_constraint_rm from model Pony',
+        )
+        # Remove constraint.
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, 'pony'].options['constraints']), 0)
+        Pony = new_state.apps.get_model(app_label, 'Pony')
+        self.assertEqual(len(Pony._meta.constraints), 0)
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        # Constraint doesn't work.
+        Pony.objects.create(pink=1, weight=4.0)
+        Pony.objects.create(pink=1, weight=4.0).delete()
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        if connection.features.supports_deferrable_unique_constraints:
+            # Unique constraint is deferred.
+            with transaction.atomic():
+                obj = Pony.objects.create(pink=1, weight=4.0)
+                obj.pink = 2
+                obj.save()
+            # Constraint behavior can be changed with SET CONSTRAINTS.
+            with self.assertRaises(IntegrityError):
+                with transaction.atomic(), connection.cursor() as cursor:
+                    quoted_name = connection.ops.quote_name(deferred_unique_constraint.name)
+                    cursor.execute('SET CONSTRAINTS %s IMMEDIATE' % quoted_name)
+                    obj = Pony.objects.create(pink=1, weight=4.0)
+                    obj.pink = 3
+                    obj.save()
+        else:
+            Pony.objects.create(pink=1, weight=4.0)
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], 'RemoveConstraint')
+        self.assertEqual(definition[1], [])
+        self.assertEqual(definition[2], {
+            'model_name': 'Pony',
+            'name': 'deferred_pink_constraint_rm',
+        })
+
     def test_alter_model_options(self):
         """
         Tests the AlterModelOptions operation.