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 095c1aaa89.

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

django/db/backends/sqlite3/introspection.py

@@ -199,6 +199,22 @@ 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
@@ -253,17 +269,5 @@ 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