Fixed #30916 -- Added support for functional unique constraints.

Thanks Ian Foote and Mariusz Felisiak for reviews.

5 files changed, 589 insertions, 2 deletions

diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index cb366d8e45..3d59d07b24 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -2,7 +2,9 @@ from unittest import mock
 
 from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection, models
+from django.db.models import F
 from django.db.models.constraints import BaseConstraint
+from django.db.models.functions import Lower
 from django.db.transaction import atomic
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 
@@ -25,6 +27,10 @@ class BaseConstraintTests(SimpleTestCase):
         with self.assertRaisesMessage(NotImplementedError, msg):
             c.constraint_sql(None, None)
 
+    def test_contains_expressions(self):
+        c = BaseConstraint('name')
+        self.assertIs(c.contains_expressions, False)
+
     def test_create_sql(self):
         c = BaseConstraint('name')
         msg = 'This method must be implemented by a subclass.'
@@ -218,6 +224,25 @@ class UniqueConstraintTests(TestCase):
         self.assertEqual(constraint_1, constraint_1)
         self.assertNotEqual(constraint_1, constraint_2)
 
+    def test_eq_with_expressions(self):
+        constraint = models.UniqueConstraint(
+            Lower('title'),
+            F('author'),
+            name='book_func_uq',
+        )
+        same_constraint = models.UniqueConstraint(
+            Lower('title'),
+            'author',
+            name='book_func_uq',
+        )
+        another_constraint = models.UniqueConstraint(
+            Lower('title'),
+            name='book_func_uq',
+        )
+        self.assertEqual(constraint, same_constraint)
+        self.assertEqual(constraint, mock.ANY)
+        self.assertNotEqual(constraint, another_constraint)
+
     def test_repr(self):
         fields = ['foo', 'bar']
         name = 'unique_fields'
@@ -275,6 +300,18 @@ class UniqueConstraintTests(TestCase):
             "opclasses=['text_pattern_ops', 'varchar_pattern_ops']>",
         )
 
+    def test_repr_with_expressions(self):
+        constraint = models.UniqueConstraint(
+            Lower('title'),
+            F('author'),
+            name='book_func_uq',
+        )
+        self.assertEqual(
+            repr(constraint),
+            "<UniqueConstraint: expressions=(Lower(F(title)), F(author)) "
+            "name='book_func_uq'>",
+        )
+
     def test_deconstruction(self):
         fields = ['foo', 'bar']
         name = 'unique_fields'
@@ -339,6 +376,14 @@ class UniqueConstraintTests(TestCase):
             'opclasses': opclasses,
         })
 
+    def test_deconstruction_with_expressions(self):
+        name = 'unique_fields'
+        constraint = models.UniqueConstraint(Lower('title'), name=name)
+        path, args, kwargs = constraint.deconstruct()
+        self.assertEqual(path, 'django.db.models.UniqueConstraint')
+        self.assertEqual(args, (Lower('title'),))
+        self.assertEqual(kwargs, {'name': name})
+
     def test_database_constraint(self):
         with self.assertRaises(IntegrityError):
             UniqueConstraintProduct.objects.create(name=self.p1.name, color=self.p1.color)
@@ -434,6 +479,15 @@ class UniqueConstraintTests(TestCase):
                 deferrable=models.Deferrable.DEFERRED,
             )
 
+    def test_deferrable_with_expressions(self):
+        message = 'UniqueConstraint with expressions cannot be deferred.'
+        with self.assertRaisesMessage(ValueError, message):
+            models.UniqueConstraint(
+                Lower('name'),
+                name='deferred_expression_unique',
+                deferrable=models.Deferrable.DEFERRED,
+            )
+
     def test_invalid_defer_argument(self):
         message = 'UniqueConstraint.deferrable must be a Deferrable instance.'
         with self.assertRaisesMessage(ValueError, message):
@@ -481,3 +535,33 @@ class UniqueConstraintTests(TestCase):
                 fields=['field'],
                 opclasses=['foo', 'bar'],
             )
+
+    def test_requires_field_or_expression(self):
+        msg = (
+            'At least one field or expression is required to define a unique '
+            'constraint.'
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            models.UniqueConstraint(name='name')
+
+    def test_expressions_and_fields_mutually_exclusive(self):
+        msg = 'UniqueConstraint.fields and expressions are mutually exclusive.'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.UniqueConstraint(Lower('field_1'), fields=['field_2'], name='name')
+
+    def test_expressions_with_opclasses(self):
+        msg = (
+            'UniqueConstraint.opclasses cannot be used with expressions. Use '
+            'django.contrib.postgres.indexes.OpClass() instead.'
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            models.UniqueConstraint(
+                Lower('field'),
+                name='test_func_opclass',
+                opclasses=['jsonb_path_ops'],
+            )
+
+    def test_requires_name(self):
+        msg = 'A unique constraint must be named.'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.UniqueConstraint(fields=['field'])
diff --git a/tests/invalid_models_tests/test_models.py b/tests/invalid_models_tests/test_models.py
index b20eef7159..c79684487d 100644
--- a/tests/invalid_models_tests/test_models.py
+++ b/tests/invalid_models_tests/test_models.py
@@ -2178,3 +2178,144 @@ class ConstraintsTests(TestCase):
                 ]
 
         self.assertEqual(Model.check(databases=self.databases), [])
+
+    def test_func_unique_constraint(self):
+        class Model(models.Model):
+            name = models.CharField(max_length=10)
+
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(Lower('name'), name='lower_name_uq'),
+                ]
+
+        warn = Warning(
+            '%s does not support unique constraints on expressions.'
+            % connection.display_name,
+            hint=(
+                "A constraint won't be created. Silence this warning if you "
+                "don't care about it."
+            ),
+            obj=Model,
+            id='models.W044',
+        )
+        expected = [] if connection.features.supports_expression_indexes else [warn]
+        self.assertEqual(Model.check(databases=self.databases), expected)
+
+    def test_func_unique_constraint_required_db_features(self):
+        class Model(models.Model):
+            name = models.CharField(max_length=10)
+
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(Lower('name'), name='lower_name_unq'),
+                ]
+                required_db_features = {'supports_expression_indexes'}
+
+        self.assertEqual(Model.check(databases=self.databases), [])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_expression_custom_lookup(self):
+        class Model(models.Model):
+            height = models.IntegerField()
+            weight = models.IntegerField()
+
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(
+                        models.F('height') / (models.F('weight__abs') + models.Value(5)),
+                        name='name',
+                    ),
+                ]
+
+        with register_lookup(models.IntegerField, Abs):
+            self.assertEqual(Model.check(databases=self.databases), [])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_pointing_to_missing_field(self):
+        class Model(models.Model):
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(Lower('missing_field').desc(), name='name'),
+                ]
+
+        self.assertEqual(Model.check(databases=self.databases), [
+            Error(
+                "'constraints' refers to the nonexistent field "
+                "'missing_field'.",
+                obj=Model,
+                id='models.E012',
+            ),
+        ])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_pointing_to_missing_field_nested(self):
+        class Model(models.Model):
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(Abs(Round('missing_field')), name='name'),
+                ]
+
+        self.assertEqual(Model.check(databases=self.databases), [
+            Error(
+                "'constraints' refers to the nonexistent field "
+                "'missing_field'.",
+                obj=Model,
+                id='models.E012',
+            ),
+        ])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_pointing_to_m2m_field(self):
+        class Model(models.Model):
+            m2m = models.ManyToManyField('self')
+
+            class Meta:
+                constraints = [models.UniqueConstraint(Lower('m2m'), name='name')]
+
+        self.assertEqual(Model.check(databases=self.databases), [
+            Error(
+                "'constraints' refers to a ManyToManyField 'm2m', but "
+                "ManyToManyFields are not permitted in 'constraints'.",
+                obj=Model,
+                id='models.E013',
+            ),
+        ])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_pointing_to_non_local_field(self):
+        class Foo(models.Model):
+            field1 = models.CharField(max_length=15)
+
+        class Bar(Foo):
+            class Meta:
+                constraints = [models.UniqueConstraint(Lower('field1'), name='name')]
+
+        self.assertEqual(Bar.check(databases=self.databases), [
+            Error(
+                "'constraints' refers to field 'field1' which is not local to "
+                "model 'Bar'.",
+                hint='This issue may be caused by multi-table inheritance.',
+                obj=Bar,
+                id='models.E016',
+            ),
+        ])
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_pointing_to_fk(self):
+        class Foo(models.Model):
+            id = models.CharField(primary_key=True, max_length=255)
+
+        class Bar(models.Model):
+            foo_1 = models.ForeignKey(Foo, models.CASCADE, related_name='bar_1')
+            foo_2 = models.ForeignKey(Foo, models.CASCADE, related_name='bar_2')
+
+            class Meta:
+                constraints = [
+                    models.UniqueConstraint(
+                        Lower('foo_1_id'),
+                        Lower('foo_2'),
+                        name='name',
+                    ),
+                ]
+
+        self.assertEqual(Bar.check(databases=self.databases), [])
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index 73a0c63921..cbd65bdb37 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -2562,6 +2562,99 @@ class OperationTests(OperationTestBase):
             'name': 'covering_pink_constraint_rm',
         })
 
+    def test_add_func_unique_constraint(self):
+        app_label = 'test_adfuncuc'
+        constraint_name = f'{app_label}_pony_abs_uq'
+        table_name = f'{app_label}_pony'
+        project_state = self.set_up_test_model(app_label)
+        constraint = models.UniqueConstraint(Abs('weight'), name=constraint_name)
+        operation = migrations.AddConstraint('Pony', constraint)
+        self.assertEqual(
+            operation.describe(),
+            'Create constraint test_adfuncuc_pony_abs_uq on model Pony',
+        )
+        self.assertEqual(
+            operation.migration_name_fragment,
+            'pony_test_adfuncuc_pony_abs_uq',
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, 'pony'].options['constraints']), 1)
+        self.assertIndexNameNotExists(table_name, constraint_name)
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        Pony = new_state.apps.get_model(app_label, 'Pony')
+        Pony.objects.create(weight=4.0)
+        if connection.features.supports_expression_indexes:
+            self.assertIndexNameExists(table_name, constraint_name)
+            with self.assertRaises(IntegrityError):
+                Pony.objects.create(weight=-4.0)
+        else:
+            self.assertIndexNameNotExists(table_name, constraint_name)
+            Pony.objects.create(weight=-4.0)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        self.assertIndexNameNotExists(table_name, constraint_name)
+        # Constraint doesn't work.
+        Pony.objects.create(weight=-4.0)
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], 'AddConstraint')
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2],
+            {'model_name': 'Pony', 'constraint': constraint},
+        )
+
+    def test_remove_func_unique_constraint(self):
+        app_label = 'test_rmfuncuc'
+        constraint_name = f'{app_label}_pony_abs_uq'
+        table_name = f'{app_label}_pony'
+        project_state = self.set_up_test_model(app_label, constraints=[
+            models.UniqueConstraint(Abs('weight'), name=constraint_name),
+        ])
+        self.assertTableExists(table_name)
+        if connection.features.supports_expression_indexes:
+            self.assertIndexNameExists(table_name, constraint_name)
+        operation = migrations.RemoveConstraint('Pony', constraint_name)
+        self.assertEqual(
+            operation.describe(),
+            'Remove constraint test_rmfuncuc_pony_abs_uq from model Pony',
+        )
+        self.assertEqual(
+            operation.migration_name_fragment,
+            'remove_pony_test_rmfuncuc_pony_abs_uq',
+        )
+        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)
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertIndexNameNotExists(table_name, constraint_name)
+        # 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_expression_indexes:
+            self.assertIndexNameExists(table_name, constraint_name)
+            with self.assertRaises(IntegrityError):
+                Pony.objects.create(weight=-4.0)
+        else:
+            self.assertIndexNameNotExists(table_name, constraint_name)
+            Pony.objects.create(weight=-4.0)
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], 'RemoveConstraint')
+        self.assertEqual(definition[1], [])
+        self.assertEqual(definition[2], {'model_name': 'Pony', 'name': constraint_name})
+
     def test_alter_model_options(self):
         """
         Tests the AlterModelOptions operation.
diff --git a/tests/postgres_tests/test_constraints.py b/tests/postgres_tests/test_constraints.py
index 80c2bb77b6..7915bb226e 100644
--- a/tests/postgres_tests/test_constraints.py
+++ b/tests/postgres_tests/test_constraints.py
@@ -1,6 +1,7 @@
 import datetime
 from unittest import mock
 
+from django.contrib.postgres.indexes import OpClass
 from django.db import (
     IntegrityError, NotSupportedError, connection, transaction,
 )
@@ -8,8 +9,8 @@ from django.db.models import (
     CheckConstraint, Deferrable, F, Func, IntegerField, Q, UniqueConstraint,
 )
 from django.db.models.fields.json import KeyTextTransform
-from django.db.models.functions import Cast, Left
-from django.test import skipUnlessDBFeature
+from django.db.models.functions import Cast, Left, Lower
+from django.test import modify_settings, skipUnlessDBFeature
 from django.utils import timezone
 
 from . import PostgreSQLTestCase
@@ -26,6 +27,7 @@ except ImportError:
     pass
 
 
+@modify_settings(INSTALLED_APPS={'append': 'django.contrib.postgres'})
 class SchemaTests(PostgreSQLTestCase):
     get_opclass_query = '''
         SELECT opcname, c.relname FROM pg_opclass AS oc
@@ -166,6 +168,33 @@ class SchemaTests(PostgreSQLTestCase):
                 [('varchar_pattern_ops', constraint.name)],
             )
 
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_opclass_func(self):
+        constraint = UniqueConstraint(
+            OpClass(Lower('scene'), name='text_pattern_ops'),
+            name='test_opclass_func',
+        )
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Scene, constraint)
+        constraints = self.get_constraints(Scene._meta.db_table)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        self.assertIn(constraint.name, constraints)
+        with editor.connection.cursor() as cursor:
+            cursor.execute(self.get_opclass_query, [constraint.name])
+            self.assertEqual(
+                cursor.fetchall(),
+                [('text_pattern_ops', constraint.name)],
+            )
+        Scene.objects.create(scene='Scene 10', setting='The dark forest of Ewing')
+        with self.assertRaises(IntegrityError), transaction.atomic():
+            Scene.objects.create(scene='ScEnE 10', setting="Sir Bedemir's Castle")
+        Scene.objects.create(scene='Scene 5', setting="Sir Bedemir's Castle")
+        # Drop the constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Scene, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(Scene._meta.db_table))
+        Scene.objects.create(scene='ScEnE 10', setting="Sir Bedemir's Castle")
+
 
 class ExclusionConstraintTests(PostgreSQLTestCase):
     def get_constraints(self, table):
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 4522f5faf4..fd8874e157 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -2189,6 +2189,246 @@ class SchemaTests(TransactionTestCase):
             AuthorWithUniqueNameAndBirthday._meta.constraints = []
             editor.remove_constraint(AuthorWithUniqueNameAndBirthday, constraint)
 
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(Upper('name').desc(), name='func_upper_uq')
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Author, constraint)
+            sql = constraint.create_sql(Author, editor)
+        table = Author._meta.db_table
+        constraints = self.get_constraints(table)
+        if connection.features.supports_index_column_ordering:
+            self.assertIndexOrder(table, constraint.name, ['DESC'])
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        # SQL contains a database function.
+        self.assertIs(sql.references_column(table, 'name'), True)
+        self.assertIn('UPPER(%s)' % editor.quote_name('name'), str(sql))
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Author, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_composite_func_unique_constraint(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(BookWithSlug)
+        constraint = UniqueConstraint(
+            Upper('title'),
+            Lower('slug'),
+            name='func_upper_lower_unq',
+        )
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(BookWithSlug, constraint)
+            sql = constraint.create_sql(BookWithSlug, editor)
+        table = BookWithSlug._meta.db_table
+        constraints = self.get_constraints(table)
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        # SQL contains database functions.
+        self.assertIs(sql.references_column(table, 'title'), True)
+        self.assertIs(sql.references_column(table, 'slug'), True)
+        sql = str(sql)
+        self.assertIn('UPPER(%s)' % editor.quote_name('title'), sql)
+        self.assertIn('LOWER(%s)' % editor.quote_name('slug'), sql)
+        self.assertLess(sql.index('UPPER'), sql.index('LOWER'))
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(BookWithSlug, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_unique_constraint_field_and_expression(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(
+            F('height').desc(),
+            'uuid',
+            Lower('name').asc(),
+            name='func_f_lower_field_unq',
+        )
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Author, constraint)
+            sql = constraint.create_sql(Author, editor)
+        table = Author._meta.db_table
+        if connection.features.supports_index_column_ordering:
+            self.assertIndexOrder(table, constraint.name, ['DESC', 'ASC', 'ASC'])
+        constraints = self.get_constraints(table)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        self.assertEqual(len(constraints[constraint.name]['columns']), 3)
+        self.assertEqual(constraints[constraint.name]['columns'][1], 'uuid')
+        # SQL contains database functions and columns.
+        self.assertIs(sql.references_column(table, 'height'), True)
+        self.assertIs(sql.references_column(table, 'name'), True)
+        self.assertIs(sql.references_column(table, 'uuid'), True)
+        self.assertIn('LOWER(%s)' % editor.quote_name('name'), str(sql))
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Author, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes', 'supports_partial_indexes')
+    def test_func_unique_constraint_partial(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(
+            Upper('name'),
+            name='func_upper_cond_weight_uq',
+            condition=Q(weight__isnull=False),
+        )
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Author, constraint)
+            sql = constraint.create_sql(Author, editor)
+        table = Author._meta.db_table
+        constraints = self.get_constraints(table)
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        self.assertIs(sql.references_column(table, 'name'), True)
+        self.assertIn('UPPER(%s)' % editor.quote_name('name'), str(sql))
+        self.assertIn(
+            'WHERE %s IS NOT NULL' % editor.quote_name('weight'),
+            str(sql),
+        )
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Author, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes', 'supports_covering_indexes')
+    def test_func_unique_constraint_covering(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(
+            Upper('name'),
+            name='func_upper_covering_uq',
+            include=['weight', 'height'],
+        )
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(Author, constraint)
+            sql = constraint.create_sql(Author, editor)
+        table = Author._meta.db_table
+        constraints = self.get_constraints(table)
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        self.assertEqual(
+            constraints[constraint.name]['columns'],
+            [None, 'weight', 'height'],
+        )
+        self.assertIs(sql.references_column(table, 'name'), True)
+        self.assertIs(sql.references_column(table, 'weight'), True)
+        self.assertIs(sql.references_column(table, 'height'), True)
+        self.assertIn('UPPER(%s)' % editor.quote_name('name'), str(sql))
+        self.assertIn(
+            'INCLUDE (%s, %s)' % (
+                editor.quote_name('weight'),
+                editor.quote_name('height'),
+            ),
+            str(sql),
+        )
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Author, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_lookups(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        with register_lookup(CharField, Lower), register_lookup(IntegerField, Abs):
+            constraint = UniqueConstraint(
+                F('name__lower'),
+                F('weight__abs'),
+                name='func_lower_abs_lookup_uq',
+            )
+            # Add constraint.
+            with connection.schema_editor() as editor:
+                editor.add_constraint(Author, constraint)
+                sql = constraint.create_sql(Author, editor)
+        table = Author._meta.db_table
+        constraints = self.get_constraints(table)
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        # SQL contains columns.
+        self.assertIs(sql.references_column(table, 'name'), True)
+        self.assertIs(sql.references_column(table, 'weight'), True)
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(Author, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_collate(self):
+        collation = connection.features.test_collations.get('non_default')
+        if not collation:
+            self.skipTest(
+                'This backend does not support case-insensitive collations.'
+            )
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(BookWithSlug)
+        constraint = UniqueConstraint(
+            Collate(F('title'), collation=collation).desc(),
+            Collate('slug', collation=collation),
+            name='func_collate_uq',
+        )
+        # Add constraint.
+        with connection.schema_editor() as editor:
+            editor.add_constraint(BookWithSlug, constraint)
+            sql = constraint.create_sql(BookWithSlug, editor)
+        table = BookWithSlug._meta.db_table
+        constraints = self.get_constraints(table)
+        self.assertIn(constraint.name, constraints)
+        self.assertIs(constraints[constraint.name]['unique'], True)
+        if connection.features.supports_index_column_ordering:
+            self.assertIndexOrder(table, constraint.name, ['DESC', 'ASC'])
+        # SQL contains columns and a collation.
+        self.assertIs(sql.references_column(table, 'title'), True)
+        self.assertIs(sql.references_column(table, 'slug'), True)
+        self.assertIn('COLLATE %s' % editor.quote_name(collation), str(sql))
+        # Remove constraint.
+        with connection.schema_editor() as editor:
+            editor.remove_constraint(BookWithSlug, constraint)
+        self.assertNotIn(constraint.name, self.get_constraints(table))
+
+    @skipIfDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_unsupported(self):
+        # UniqueConstraint is ignored on databases that don't support indexes on
+        # expressions.
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(F('name'), name='func_name_uq')
+        with connection.schema_editor() as editor, self.assertNumQueries(0):
+            self.assertIsNone(editor.add_constraint(Author, constraint))
+            self.assertIsNone(editor.remove_constraint(Author, constraint))
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_nonexistent_field(self):
+        constraint = UniqueConstraint(Lower('nonexistent'), name='func_nonexistent_uq')
+        msg = (
+            "Cannot resolve keyword 'nonexistent' into field. Choices are: "
+            "height, id, name, uuid, weight"
+        )
+        with self.assertRaisesMessage(FieldError, msg):
+            with connection.schema_editor() as editor:
+                editor.add_constraint(Author, constraint)
+
+    @skipUnlessDBFeature('supports_expression_indexes')
+    def test_func_unique_constraint_nondeterministic(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        constraint = UniqueConstraint(Random(), name='func_random_uq')
+        with connection.schema_editor() as editor:
+            with self.assertRaises(DatabaseError):
+                editor.add_constraint(Author, constraint)
+
     def test_index_together(self):
         """
         Tests removing and adding index_together constraints on a model.