Fixed #29641 -- Added support for unique constraints in Meta.constraints.

This constraint is similar to Meta.unique_together but also allows
specifying a name.

Co-authored-by: Ian Foote <python@ian.feete.org>

3 files changed, 56 insertions, 2 deletions

diff --git a/django/db/backends/sqlite3/introspection.py b/django/db/backends/sqlite3/introspection.py
index 47ca25a78a..4f4a54eacf 100644
--- a/django/db/backends/sqlite3/introspection.py
+++ b/django/db/backends/sqlite3/introspection.py
@@ -260,6 +260,15 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                     name_token = next_ttype(sqlparse.tokens.Literal.String.Symbol)
                     name = name_token.value[1:-1]
                     token = next_ttype(sqlparse.tokens.Keyword)
+                if token.match(sqlparse.tokens.Keyword, 'UNIQUE'):
+                    constraints[name] = {
+                        'unique': True,
+                        'columns': [],
+                        'primary_key': False,
+                        'foreign_key': False,
+                        'check': False,
+                        'index': False,
+                    }
                 if token.match(sqlparse.tokens.Keyword, 'CHECK'):
                     # Column check constraint
                     if name is None:
diff --git a/django/db/models/base.py b/django/db/models/base.py
index 89faf9d1e1..b57726fbcf 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -16,7 +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.constraints import CheckConstraint, UniqueConstraint
 from django.db.models.deletion import CASCADE, Collector
 from django.db.models.fields.related import (
     ForeignObjectRel, OneToOneField, lazy_related_operation, resolve_relation,
@@ -982,9 +982,12 @@ class Model(metaclass=ModelBase):
         unique_checks = []
 
         unique_togethers = [(self.__class__, self._meta.unique_together)]
+        constraints = [(self.__class__, self._meta.constraints)]
         for parent_class in self._meta.get_parent_list():
             if parent_class._meta.unique_together:
                 unique_togethers.append((parent_class, parent_class._meta.unique_together))
+            if parent_class._meta.constraints:
+                constraints.append((parent_class, parent_class._meta.constraints))
 
         for model_class, unique_together in unique_togethers:
             for check in unique_together:
@@ -992,6 +995,12 @@ class Model(metaclass=ModelBase):
                     # Add the check if the field isn't excluded.
                     unique_checks.append((model_class, tuple(check)))
 
+        for model_class, model_constraints in constraints:
+            for constraint in model_constraints:
+                if (isinstance(constraint, UniqueConstraint) and
+                        not any(name in exclude for name in constraint.fields)):
+                    unique_checks.append((model_class, constraint.fields))
+
         # These are checks for the unique_for_<date/year/month>.
         date_checks = []
 
diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 698b278fe8..f6ea0fa12a 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -1,6 +1,6 @@
 from django.db.models.sql.query import Query
 
-__all__ = ['CheckConstraint']
+__all__ = ['CheckConstraint', 'UniqueConstraint']
 
 
 class BaseConstraint:
@@ -68,3 +68,39 @@ class CheckConstraint(BaseConstraint):
         path, args, kwargs = super().deconstruct()
         kwargs['check'] = self.check
         return path, args, kwargs
+
+
+class UniqueConstraint(BaseConstraint):
+    def __init__(self, *, fields, name):
+        if not fields:
+            raise ValueError('At least one field is required to define a unique constraint.')
+        self.fields = tuple(fields)
+        super().__init__(name)
+
+    def constraint_sql(self, model, schema_editor):
+        columns = (
+            model._meta.get_field(field_name).column
+            for field_name in self.fields
+        )
+        return schema_editor.sql_unique_constraint % {
+            'columns': ', '.join(map(schema_editor.quote_name, columns)),
+        }
+
+    def create_sql(self, model, schema_editor):
+        columns = [model._meta.get_field(field_name).column for field_name in self.fields]
+        return schema_editor._create_unique_sql(model, columns, self.name)
+
+    def __repr__(self):
+        return '<%s: fields=%r name=%r>' % (self.__class__.__name__, self.fields, self.name)
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, UniqueConstraint) and
+            self.name == other.name and
+            self.fields == other.fields
+        )
+
+    def deconstruct(self):
+        path, args, kwargs = super().deconstruct()
+        kwargs['fields'] = self.fields
+        return path, args, kwargs