import changes from Django 1.11.3

Author: michiya

sql_server/pyodbc/schema.py

@@ -350,7 +350,7 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
         # Rebuild FKs that pointed to us if we previously had to drop them
         if old_field.primary_key and new_field.primary_key and old_type != new_type:
             for rel in new_field.model._meta.related_objects:
-                if not rel.many_to_many:
+                if not rel.many_to_many and rel.field.db_constraint:
                     self.execute(self._create_fk_sql(rel.related_model, rel.field, "_fk"))
         # Does it have check constraints we need to add?
         if old_db_params['check'] != new_db_params['check'] and new_db_params['check']: