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>

7 files changed, 126 insertions, 5 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
diff --git a/docs/ref/models/constraints.txt b/docs/ref/models/constraints.txt
index 9e24a1cad8..5c94ce094a 100644
--- a/docs/ref/models/constraints.txt
+++ b/docs/ref/models/constraints.txt
@@ -43,3 +43,28 @@ ensures the age field is never less than 18.
 .. attribute:: CheckConstraint.name
 
 The name of the constraint.
+
+``UniqueConstraint``
+====================
+
+.. class:: UniqueConstraint(*, fields, name)
+
+    Creates a unique constraint in the database.
+
+``fields``
+----------
+
+.. attribute:: UniqueConstraint.fields
+
+A list of field names that specifies the unique set of columns you want the
+constraint to enforce.
+
+For example ``UniqueConstraint(fields=['room', 'date'], name='unique_location')``
+ensures only one location can exist for each ``date``.
+
+``name``
+--------
+
+.. attribute:: UniqueConstraint.name
+
+The name of the constraint.
diff --git a/docs/releases/2.2.txt b/docs/releases/2.2.txt
index ef68c31eab..8a7a75a0bc 100644
--- a/docs/releases/2.2.txt
+++ b/docs/releases/2.2.txt
@@ -33,7 +33,8 @@ What's new in Django 2.2
 Constraints
 -----------
 
-The new :class:`~django.db.models.CheckConstraint` class enables adding custom
+The new :class:`~django.db.models.CheckConstraint` and
+:class:`~django.db.models.UniqueConstraint` classes enable adding custom
 database constraints. Constraints are added to models using the
 :attr:`Meta.constraints <django.db.models.Options.constraints>` option.
 
diff --git a/tests/constraints/models.py b/tests/constraints/models.py
index 08fbe9e1df..b1645ecedb 100644
--- a/tests/constraints/models.py
+++ b/tests/constraints/models.py
@@ -3,8 +3,8 @@ from django.db import models
 
 class Product(models.Model):
     name = models.CharField(max_length=255)
-    price = models.IntegerField()
-    discounted_price = models.IntegerField()
+    price = models.IntegerField(null=True)
+    discounted_price = models.IntegerField(null=True)
 
     class Meta:
         constraints = [
@@ -12,4 +12,5 @@ class Product(models.Model):
                 check=models.Q(price__gt=models.F('discounted_price')),
                 name='price_gt_discounted_price',
             ),
+            models.UniqueConstraint(fields=['name'], name='unique_name'),
         ]
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index b144c24a28..ddcaf7cfe0 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -1,3 +1,4 @@
+from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection, models
 from django.db.models.constraints import BaseConstraint
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
@@ -50,3 +51,42 @@ class CheckConstraintTests(TestCase):
         if connection.features.uppercases_column_names:
             expected_name = expected_name.upper()
         self.assertIn(expected_name, constraints)
+
+
+class UniqueConstraintTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.p1 = Product.objects.create(name='p1')
+
+    def test_repr(self):
+        fields = ['foo', 'bar']
+        name = 'unique_fields'
+        constraint = models.UniqueConstraint(fields=fields, name=name)
+        self.assertEqual(
+            repr(constraint),
+            "<UniqueConstraint: fields=('foo', 'bar') name='unique_fields'>",
+        )
+
+    def test_deconstruction(self):
+        fields = ['foo', 'bar']
+        name = 'unique_fields'
+        check = models.UniqueConstraint(fields=fields, name=name)
+        path, args, kwargs = check.deconstruct()
+        self.assertEqual(path, 'django.db.models.UniqueConstraint')
+        self.assertEqual(args, ())
+        self.assertEqual(kwargs, {'fields': tuple(fields), 'name': name})
+
+    def test_database_constraint(self):
+        with self.assertRaises(IntegrityError):
+            Product.objects.create(name=self.p1.name)
+
+    def test_model_validation(self):
+        with self.assertRaisesMessage(ValidationError, 'Product with this Name already exists.'):
+            Product(name=self.p1.name).validate_unique()
+
+    def test_name(self):
+        constraints = get_constraints(Product._meta.db_table)
+        expected_name = 'unique_name'
+        if connection.features.uppercases_column_names:
+            expected_name = expected_name.upper()
+        self.assertIn(expected_name, constraints)