diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a57e37c
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.idea
+.pyc
\ No newline at end of file
diff --git a/django_db_constraints/autodetector.py b/django_db_constraints/autodetector.py
index a21d409..ecb1a9b 100644
--- a/django_db_constraints/autodetector.py
+++ b/django_db_constraints/autodetector.py
@@ -8,7 +8,7 @@ class MigrationAutodetectorWithDbConstraints(MigrationAutodetector):
     db_constraints_operations = []
 
     def generate_created_models(self, *args, **kwargs):
-        rv = super().generate_created_models(*args, **kwargs)
+        rv = super(MigrationAutodetectorWithDbConstraints, self).generate_created_models(*args, **kwargs)
         for (app_label, migration_operations) in self.generated_operations.items():
             for operation in migration_operations:
                 if isinstance(operation, operations.CreateModel) and 'db_constraints' in operation.options:
@@ -20,7 +20,7 @@ def generate_created_models(self, *args, **kwargs):
         return rv
 
     def generate_altered_unique_together(self, *args, **kwargs):
-        rv = super().generate_altered_unique_together(*args, **kwargs)
+        rv = super(MigrationAutodetectorWithDbConstraints, self).generate_altered_unique_together(*args, **kwargs)
 
         for app_label, model_name in sorted(self.kept_model_keys):
             old_model_name = self.renamed_models.get((app_label, model_name), model_name)
@@ -41,7 +41,7 @@ def generate_altered_unique_together(self, *args, **kwargs):
         return rv
 
     def _sort_migrations(self, *args, **kwargs):
-        rv = super()._sort_migrations()
+        rv = super(MigrationAutodetectorWithDbConstraints, self)._sort_migrations()
         for app_label, operation in self.db_constraints_operations:
             self.generated_operations.setdefault(app_label, []).append(operation)
         return rv
diff --git a/django_db_constraints/operations.py b/django_db_constraints/operations.py
index 933cef1..8bd851f 100644
--- a/django_db_constraints/operations.py
+++ b/django_db_constraints/operations.py
@@ -11,7 +11,7 @@ class AlterConstraints(ModelOptionOperation):
 
     def __init__(self, name, db_constraints):
         self.db_constraints = db_constraints
-        super().__init__(name)
+        super(AlterConstraints, self).__init__(name)
 
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.name_lower]
@@ -24,8 +24,8 @@ def database_forwards(self, app_label, schema_editor, from_state, to_state):
         if self.allow_migrate_model(schema_editor.connection.alias, to_model):
             from_model = from_state.apps.get_model(app_label, self.name)
 
-            to_constraints = getattr(to_model._meta, self.option_name, {}).keys()
-            from_constraints = getattr(from_model._meta, self.option_name, {}).keys()
+            to_constraints = set(getattr(to_model._meta, self.option_name, {}).keys())
+            from_constraints = set(getattr(from_model._meta, self.option_name, {}).keys())
 
             table_operations = tuple(
                 'DROP CONSTRAINT IF EXISTS {name}'.format(
@@ -38,19 +38,22 @@ def database_forwards(self, app_label, schema_editor, from_state, to_state):
                     constraint=to_model._meta.db_constraints[constraint_name],
                 )
                 for constraint_name in to_constraints - from_constraints
-            ) + tuple(
-                'DROP CONSTRAINT IF EXISTS {name}, ADD CONSTRAINT {name} {constraint}'.format(
-                    name=schema_editor.connection.ops.quote_name(constraint_name),
-                    constraint=to_model._meta.db_constraints[constraint_name],
-                )
-                for constraint_name in to_constraints & from_constraints
-                if to_model._meta.db_constraints[constraint_name] != from_model._meta.db_constraints[constraint_name]
             )
-
-            if table_operations:
-                schema_editor.execute('ALTER TABLE {table} {table_operations}'.format(
+            for constraint_name in to_constraints & from_constraints:
+                if to_model._meta.db_constraints[constraint_name] != from_model._meta.db_constraints[constraint_name]:
+                    table_operations += (
+                        'DROP CONSTRAINT IF EXISTS {name}'.format(
+                            name=schema_editor.connection.ops.quote_name(constraint_name)
+                        ),
+                        'ADD CONSTRAINT {name} {constraint}'.format(
+                            name=schema_editor.connection.ops.quote_name(constraint_name),
+                            constraint=to_model._meta.db_constraints[constraint_name],
+                        )
+                    )
+            for table_operation in table_operations:
+                schema_editor.execute('ALTER TABLE {table} {table_operation}'.format(
                     table=schema_editor.connection.ops.quote_name(to_model._meta.db_table),
-                    table_operations=', '.join(table_operations),
+                    table_operation=table_operation,
                 ))
 
     def database_backwards(self, app_label, schema_editor, from_state, to_state):
diff --git a/setup.py b/setup.py
index ff3bfb8..55f6f32 100644
--- a/setup.py
+++ b/setup.py
@@ -6,7 +6,7 @@ def readme():
 
 setup(
     name='django-db-constraints',
-    version='0.3.0',
+    version='0.4.0',
     author='shangxiao',
     description='Add database table-level constraints to your Django model\'s Meta',
     long_description=readme(),