[2.2.x] Refs #30172 -- Prevented removing a model Meta's index/unique_together from removing Meta constraints/indexes.

Backport of 5c17c273ae from master.

django/db/backends/base/schema.py

@@ -383,8 +383,13 @@ class BaseDatabaseSchemaEditor:
             self.execute(self._create_index_sql(model, fields, suffix="_idx"))
 
     def _delete_composed_index(self, model, fields, constraint_kwargs, sql):
+        meta_constraint_names = {constraint.name for constraint in model._meta.constraints}
+        meta_index_names = {constraint.name for constraint in model._meta.indexes}
         columns = [model._meta.get_field(field).column for field in fields]
-        constraint_names = self._constraint_names(model, columns, **constraint_kwargs)
+        constraint_names = self._constraint_names(
+            model, columns, exclude=meta_constraint_names | meta_index_names,
+            **constraint_kwargs
+        )
         if len(constraint_names) != 1:
             raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                 len(constraint_names),
@@ -593,9 +598,11 @@ class BaseDatabaseSchemaEditor:
             meta_index_names = {index.name for index in model._meta.indexes}
             # Retrieve only BTREE indexes since this is what's created with
             # db_index=True.
-            index_names = self._constraint_names(model, [old_field.column], index=True, type_=Index.suffix)
+            index_names = self._constraint_names(
+                model, [old_field.column], index=True, type_=Index.suffix,
+                exclude=meta_index_names,
+            )
             for index_name in index_names:
-                if index_name not in meta_index_names:
                 # The only way to check if an index was created with
                 # db_index=True or with Index(['field'], name='foo')
                 # is to look at its name (refs #28053).

tests/schema/models.py

@@ -62,6 +62,24 @@ class AuthorWithUniqueName(models.Model):
         apps = new_apps
 
 
+class AuthorWithIndexedNameAndBirthday(models.Model):
+    name = models.CharField(max_length=255)
+    birthday = models.DateField()
+
+    class Meta:
+        apps = new_apps
+        index_together = [['name', 'birthday']]
+
+
+class AuthorWithUniqueNameAndBirthday(models.Model):
+    name = models.CharField(max_length=255)
+    birthday = models.DateField()
+
+    class Meta:
+        apps = new_apps
+        unique_together = [['name', 'birthday']]
+
+
 class Book(models.Model):
     author = models.ForeignKey(Author, models.CASCADE)
     title = models.CharField(max_length=100, db_index=True)

tests/schema/tests.py

@@ -32,10 +32,11 @@ from .fields import (
 from .models import (
     Author, AuthorCharFieldWithIndex, AuthorTextFieldWithIndex,
     AuthorWithDefaultHeight, AuthorWithEvenLongerName, AuthorWithIndexedName,
-    AuthorWithUniqueName, Book, BookForeignObj, BookWeak, BookWithLongName,
-    BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK, Node, Note,
-    NoteRename, Tag, TagIndexed, TagM2MTest, TagUniqueRename, Thing,
-    UniqueTest, new_apps,
+    AuthorWithIndexedNameAndBirthday, AuthorWithUniqueName,
+    AuthorWithUniqueNameAndBirthday, Book, BookForeignObj, BookWeak,
+    BookWithLongName, BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK,
+    Node, Note, NoteRename, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
+    Thing, UniqueTest, new_apps,
 )
 
 
@@ -1818,6 +1819,50 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_unique_together(Book, [['author', 'title']], [])
 
+    @skipUnlessDBFeature('allows_multiple_constraints_on_same_fields')
+    def test_remove_unique_together_does_not_remove_meta_constraints(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithUniqueNameAndBirthday)
+        # Add the custom unique constraint
+        constraint = UniqueConstraint(fields=['name', 'birthday'], name='author_name_birthday_uniq')
+        custom_constraint_name = constraint.name
+        AuthorWithUniqueNameAndBirthday._meta.constraints = [constraint]
+        with connection.schema_editor() as editor:
+            editor.add_constraint(AuthorWithUniqueNameAndBirthday, constraint)
+        # Ensure the constraints exist
+        constraints = self.get_constraints(AuthorWithUniqueNameAndBirthday._meta.db_table)
+        self.assertIn(custom_constraint_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['unique'] and name != custom_constraint_name
+        ]
+        self.assertEqual(len(other_constraints), 1)
+        # Remove unique together
+        unique_together = AuthorWithUniqueNameAndBirthday._meta.unique_together
+        with connection.schema_editor() as editor:
+            editor.alter_unique_together(AuthorWithUniqueNameAndBirthday, unique_together, [])
+        constraints = self.get_constraints(AuthorWithUniqueNameAndBirthday._meta.db_table)
+        self.assertIn(custom_constraint_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['unique'] and name != custom_constraint_name
+        ]
+        self.assertEqual(len(other_constraints), 0)
+        # Re-add unique together
+        with connection.schema_editor() as editor:
+            editor.alter_unique_together(AuthorWithUniqueNameAndBirthday, [], unique_together)
+        constraints = self.get_constraints(AuthorWithUniqueNameAndBirthday._meta.db_table)
+        self.assertIn(custom_constraint_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['unique'] and name != custom_constraint_name
+        ]
+        self.assertEqual(len(other_constraints), 1)
+        # Drop the unique constraint
+        with connection.schema_editor() as editor:
+            AuthorWithUniqueNameAndBirthday._meta.constraints = []
+            editor.remove_constraint(AuthorWithUniqueNameAndBirthday, constraint)
+
     def test_index_together(self):
         """
         Tests removing and adding index_together constraints on a model.
@@ -1896,6 +1941,50 @@ class SchemaTests(TransactionTestCase):
             ),
         )
 
+    @skipUnlessDBFeature('allows_multiple_constraints_on_same_fields')
+    def test_remove_index_together_does_not_remove_meta_indexes(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithIndexedNameAndBirthday)
+        # Add the custom index
+        index = Index(fields=['name', 'birthday'], name='author_name_birthday_idx')
+        custom_index_name = index.name
+        AuthorWithIndexedNameAndBirthday._meta.indexes = [index]
+        with connection.schema_editor() as editor:
+            editor.add_index(AuthorWithIndexedNameAndBirthday, index)
+        # Ensure the indexes exist
+        constraints = self.get_constraints(AuthorWithIndexedNameAndBirthday._meta.db_table)
+        self.assertIn(custom_index_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['index'] and name != custom_index_name
+        ]
+        self.assertEqual(len(other_constraints), 1)
+        # Remove index together
+        index_together = AuthorWithIndexedNameAndBirthday._meta.index_together
+        with connection.schema_editor() as editor:
+            editor.alter_index_together(AuthorWithIndexedNameAndBirthday, index_together, [])
+        constraints = self.get_constraints(AuthorWithIndexedNameAndBirthday._meta.db_table)
+        self.assertIn(custom_index_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['index'] and name != custom_index_name
+        ]
+        self.assertEqual(len(other_constraints), 0)
+        # Re-add index together
+        with connection.schema_editor() as editor:
+            editor.alter_index_together(AuthorWithIndexedNameAndBirthday, [], index_together)
+        constraints = self.get_constraints(AuthorWithIndexedNameAndBirthday._meta.db_table)
+        self.assertIn(custom_index_name, constraints)
+        other_constraints = [
+            name for name, details in constraints.items()
+            if details['columns'] == ['name', 'birthday'] and details['index'] and name != custom_index_name
+        ]
+        self.assertEqual(len(other_constraints), 1)
+        # Drop the index
+        with connection.schema_editor() as editor:
+            AuthorWithIndexedNameAndBirthday._meta.indexes = []
+            editor.remove_index(AuthorWithIndexedNameAndBirthday, index)
+
     @isolate_apps('schema')
     def test_db_table(self):
         """