[2.0.x] Fixed #28884 -- Fixed crash on SQLite when renaming a field in a model referenced by a ManyToManyField.

Introspected database constraints instead of relying on _meta.related_objects
to determine whether or not a table or a column is referenced on rename
operations.

This has the side effect of ignoring both db_constraint=False and virtual
fields such as GenericRelation which aren't backend by database level
constraints and thus shouldn't prevent the rename operations from being
performed in a transaction.

Regression in 095c1aaa898bed40568009db836aa8434f1b983d.

Thanks Tim for the additional tests and edits, and Mariusz for the review.

Backport of 9f7772e098439f9edea3d25ab127539fc514eeb2 from master

5 files changed, 146 insertions, 21 deletions

diff --git a/django/db/backends/sqlite3/introspection.py b/django/db/backends/sqlite3/introspection.py
index 0518641344..de6d6da465 100644
--- a/django/db/backends/sqlite3/introspection.py
+++ b/django/db/backends/sqlite3/introspection.py
@@ -239,6 +239,22 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             'pk': field[5],  # undocumented
         } for field in cursor.fetchall()]
 
+    def _get_foreign_key_constraints(self, cursor, table_name):
+        constraints = {}
+        cursor.execute('PRAGMA foreign_key_list(%s)' % self.connection.ops.quote_name(table_name))
+        for row in cursor.fetchall():
+            # Remaining on_update/on_delete/match values are of no interest.
+            id_, _, table, from_, to = row[:5]
+            constraints['fk_%d' % id_] = {
+                'columns': [from_],
+                'primary_key': False,
+                'unique': False,
+                'foreign_key': (table, to),
+                'check': False,
+                'index': False,
+            }
+        return constraints
+
     def get_constraints(self, cursor, table_name):
         """
         Retrieve any constraints or keys (unique, pk, fk, check, index) across
@@ -293,17 +309,5 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                 "check": False,
                 "index": False,
             }
-        # Get foreign keys
-        cursor.execute('PRAGMA foreign_key_list(%s)' % self.connection.ops.quote_name(table_name))
-        for row in cursor.fetchall():
-            # Remaining on_update/on_delete/match values are of no interest here
-            id_, seq, table, from_, to = row[:5]
-            constraints['fk_%d' % id_] = {
-                'columns': [from_],
-                'primary_key': False,
-                'unique': False,
-                'foreign_key': (table, to),
-                'check': False,
-                'index': False,
-            }
+        constraints.update(self._get_foreign_key_constraints(cursor, table_name))
         return constraints
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index 8c427bee45..add05843c3 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -61,8 +61,27 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         else:
             raise ValueError("Cannot quote parameter value %r of type %s" % (value, type(value)))
 
+    def _is_referenced_by_fk_constraint(self, table_name, column_name=None, ignore_self=False):
+        """
+        Return whether or not the provided table name is referenced by another
+        one. If `column_name` is specified, only references pointing to that
+        column are considered. If `ignore_self` is True, self-referential
+        constraints are ignored.
+        """
+        with self.connection.cursor() as cursor:
+            for other_table in self.connection.introspection.get_table_list(cursor):
+                if ignore_self and other_table.name == table_name:
+                    continue
+                constraints = self.connection.introspection._get_foreign_key_constraints(cursor, other_table.name)
+                for constraint in constraints.values():
+                    constraint_table, constraint_column = constraint['foreign_key']
+                    if (constraint_table == table_name and
+                            (column_name is None or constraint_column == column_name)):
+                        return True
+        return False
+
     def alter_db_table(self, model, old_db_table, new_db_table, disable_constraints=True):
-        if model._meta.related_objects and disable_constraints:
+        if disable_constraints and self._is_referenced_by_fk_constraint(old_db_table):
             if self.connection.in_atomic_block:
                 raise NotSupportedError((
                     'Renaming the %r table while in a transaction is not '
@@ -77,8 +96,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def alter_field(self, model, old_field, new_field, strict=False):
         old_field_name = old_field.name
+        table_name = model._meta.db_table
+        _, old_column_name = old_field.get_attname_column()
         if (new_field.name != old_field_name and
-                any(r.field_name == old_field.name for r in model._meta.related_objects)):
+                self._is_referenced_by_fk_constraint(table_name, old_column_name, ignore_self=True)):
             if self.connection.in_atomic_block:
                 raise NotSupportedError((
                     'Renaming the %r.%r column while in a transaction is not '
@@ -93,9 +114,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 with self.connection.cursor() as cursor:
                     schema_version = cursor.execute('PRAGMA schema_version').fetchone()[0]
                     cursor.execute('PRAGMA writable_schema = 1')
-                    table_name = model._meta.db_table
                     references_template = ' REFERENCES "%s" ("%%s") ' % table_name
-                    old_column_name = old_field.get_attname_column()[1]
                     new_column_name = new_field.get_attname_column()[1]
                     search = references_template % old_column_name
                     replacement = references_template % new_column_name
diff --git a/docs/releases/2.0.1.txt b/docs/releases/2.0.1.txt
index 19164dab8a..ccbc7c29a3 100644
--- a/docs/releases/2.0.1.txt
+++ b/docs/releases/2.0.1.txt
@@ -34,3 +34,6 @@ Bugfixes
 
 * Fixed crash when coercing a translatable URL pattern to ``str``
   (:ticket:`28947`).
+
+* Fixed crash on SQLite when renaming a field in a model referenced by a
+  ``ManyToManyField`` (:ticket:`28884`).
diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index 2e24047d6d..d953ea20aa 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -722,7 +722,7 @@ class OperationTests(OperationTestBase):
 
         project_state = self.apply_operations(app_label, project_state, operations=[
             migrations.RenameModel("Pony", "Pony2"),
-        ])
+        ], atomic=connection.features.supports_atomic_references_rename)
         Pony = project_state.apps.get_model(app_label, "Pony2")
         Rider = project_state.apps.get_model(app_label, "Rider")
         pony = Pony.objects.create()
@@ -1231,12 +1231,13 @@ class OperationTests(OperationTestBase):
         second_state = first_state.clone()
         operation = migrations.AlterModelTable(name='pony', table=None)
         operation.state_forwards(app_label, second_state)
-        with connection.schema_editor() as editor:
+        atomic_rename = connection.features.supports_atomic_references_rename
+        with connection.schema_editor(atomic=atomic_rename) as editor:
             operation.database_forwards(app_label, editor, first_state, second_state)
         self.assertTableExists(new_m2m_table)
         self.assertTableNotExists(original_m2m_table)
         # And test reversal
-        with connection.schema_editor() as editor:
+        with connection.schema_editor(atomic=atomic_rename) as editor:
             operation.database_backwards(app_label, editor, second_state, first_state)
         self.assertTableExists(original_m2m_table)
         self.assertTableNotExists(new_m2m_table)
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 4781a6bb3c..1fa715da51 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -61,6 +61,9 @@ class SchemaTests(TransactionTestCase):
         # local_models should contain test dependent model classes that will be
         # automatically removed from the app cache on test tear down.
         self.local_models = []
+        # isolated_local_models contains models that are in test methods
+        # decorated with @isolate_apps.
+        self.isolated_local_models = []
 
     def tearDown(self):
         # Delete any tables made for our models
@@ -75,6 +78,10 @@ class SchemaTests(TransactionTestCase):
                     if through and through._meta.auto_created:
                         del new_apps.all_models['schema'][through._meta.model_name]
                 del new_apps.all_models['schema'][model._meta.model_name]
+        if self.isolated_local_models:
+            with connection.schema_editor() as editor:
+                for model in self.isolated_local_models:
+                    editor.delete_model(model)
 
     def delete_tables(self):
         "Deletes all model tables for our models for a clean test environment"
@@ -1420,6 +1427,38 @@ class SchemaTests(TransactionTestCase):
     def test_m2m_repoint_inherited(self):
         self._test_m2m_repoint(InheritedManyToManyField)
 
+    @isolate_apps('schema')
+    def test_m2m_rename_field_in_target_model(self):
+        class TagM2MTest(Model):
+            title = CharField(max_length=255)
+
+            class Meta:
+                app_label = 'schema'
+
+        class LocalM2M(Model):
+            tags = ManyToManyField(TagM2MTest)
+
+            class Meta:
+                app_label = 'schema'
+
+        # Create the tables.
+        with connection.schema_editor() as editor:
+            editor.create_model(LocalM2M)
+            editor.create_model(TagM2MTest)
+        # Ensure the m2m table is there.
+        self.assertEqual(len(self.column_classes(LocalM2M)), 1)
+        # Alter a field in TagM2MTest.
+        old_field = TagM2MTest._meta.get_field('title')
+        new_field = CharField(max_length=254)
+        new_field.contribute_to_class(TagM2MTest, 'title1')
+        # @isolate_apps() and inner models are needed to have the model
+        # relations populated, otherwise this doesn't act as a regression test.
+        self.assertEqual(len(new_field.model._meta.related_objects), 1)
+        with connection.schema_editor() as editor:
+            editor.alter_field(TagM2MTest, old_field, new_field, strict=True)
+        # Ensure the m2m table is still there.
+        self.assertEqual(len(self.column_classes(LocalM2M)), 1)
+
     @skipUnlessDBFeature('supports_column_check_constraints')
     def test_check_constraints(self):
         """
@@ -2373,6 +2412,7 @@ class SchemaTests(TransactionTestCase):
         new_field.set_attributes_from_name('id')
         with connection.schema_editor() as editor:
             editor.alter_field(Node, old_field, new_field, strict=True)
+        self.assertForeignKeyExists(Node, 'parent_id', Node._meta.db_table)
 
     @mock.patch('django.db.backends.base.schema.datetime')
     @mock.patch('django.db.backends.base.schema.timezone')
@@ -2481,7 +2521,8 @@ class SchemaTests(TransactionTestCase):
         doc.students.add(student)
 
     def test_rename_table_renames_deferred_sql_references(self):
-        with connection.schema_editor() as editor:
+        atomic_rename = connection.features.supports_atomic_references_rename
+        with connection.schema_editor(atomic=atomic_rename) as editor:
             editor.create_model(Author)
             editor.create_model(Book)
             editor.alter_db_table(Author, 'schema_author', 'schema_renamed_author')
@@ -2508,3 +2549,60 @@ class SchemaTests(TransactionTestCase):
             for statement in editor.deferred_sql:
                 self.assertIs(statement.references_column('book', 'title'), False)
                 self.assertIs(statement.references_column('book', 'author_id'), False)
+
+    @isolate_apps('schema')
+    def test_referenced_field_without_constraint_rename_inside_atomic_block(self):
+        """
+        Foreign keys without database level constraint don't prevent the field
+        they reference from being renamed in an atomic block.
+        """
+        class Foo(Model):
+            field = CharField(max_length=255, unique=True)
+
+            class Meta:
+                app_label = 'schema'
+
+        class Bar(Model):
+            foo = ForeignKey(Foo, CASCADE, to_field='field', db_constraint=False)
+
+            class Meta:
+                app_label = 'schema'
+
+        self.isolated_local_models = [Foo, Bar]
+        with connection.schema_editor() as editor:
+            editor.create_model(Foo)
+            editor.create_model(Bar)
+
+        new_field = CharField(max_length=255, unique=True)
+        new_field.set_attributes_from_name('renamed')
+        with connection.schema_editor(atomic=True) as editor:
+            editor.alter_field(Foo, Foo._meta.get_field('field'), new_field)
+
+    @isolate_apps('schema')
+    def test_referenced_table_without_constraint_rename_inside_atomic_block(self):
+        """
+        Foreign keys without database level constraint don't prevent the table
+        they reference from being renamed in an atomic block.
+        """
+        class Foo(Model):
+            field = CharField(max_length=255, unique=True)
+
+            class Meta:
+                app_label = 'schema'
+
+        class Bar(Model):
+            foo = ForeignKey(Foo, CASCADE, to_field='field', db_constraint=False)
+
+            class Meta:
+                app_label = 'schema'
+
+        self.isolated_local_models = [Foo, Bar]
+        with connection.schema_editor() as editor:
+            editor.create_model(Foo)
+            editor.create_model(Bar)
+
+        new_field = CharField(max_length=255, unique=True)
+        new_field.set_attributes_from_name('renamed')
+        with connection.schema_editor(atomic=True) as editor:
+            editor.alter_db_table(Foo, Foo._meta.db_table, 'renamed_table')
+        Foo._meta.db_table = 'renamed_table'