Fixed #25833 -- Added support for non-atomic migrations.

Added the Migration.atomic attribute which can be set to False
for non-atomic migrations.

django/core/management/commands/sqlmigrate.py

@@ -51,6 +51,9 @@ class Command(BaseCommand):
                 migration_name, app_label))
         targets = [(app_label, migration.name)]
 
+        # Show begin/end around output only for atomic migrations
+        self.output_transaction = migration.atomic
+
         # Make a plan that represents just the requested migrations and show SQL
         # for it
         plan = [(executor.loader.graph.nodes[targets[0]], options['backwards'])]

django/db/backends/base/schema.py

@@ -69,17 +69,18 @@ class BaseDatabaseSchemaEditor(object):
     sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
     sql_delete_pk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
 
-    def __init__(self, connection, collect_sql=False):
+    def __init__(self, connection, collect_sql=False, atomic=True):
         self.connection = connection
         self.collect_sql = collect_sql
         if self.collect_sql:
             self.collected_sql = []
+        self.atomic_migration = self.connection.features.can_rollback_ddl and atomic
 
     # State-managing methods
 
     def __enter__(self):
         self.deferred_sql = []
-        if self.connection.features.can_rollback_ddl:
+        if self.atomic_migration:
             self.atomic = atomic(self.connection.alias)
             self.atomic.__enter__()
         return self
@@ -88,7 +89,7 @@ class BaseDatabaseSchemaEditor(object):
         if exc_type is None:
             for sql in self.deferred_sql:
                 self.execute(sql)
-        if self.connection.features.can_rollback_ddl:
+        if self.atomic_migration:
             self.atomic.__exit__(exc_type, exc_value, traceback)
 
     # Core utility functions

django/db/migrations/executor.py

@@ -170,7 +170,7 @@ class MigrationExecutor(object):
         statements = []
         state = None
         for migration, backwards in plan:
-            with self.connection.schema_editor(collect_sql=True) as schema_editor:
+            with self.connection.schema_editor(collect_sql=True, atomic=migration.atomic) as schema_editor:
                 if state is None:
                     state = self.loader.project_state((migration.app_label, migration.name), at_end=False)
                 if not backwards:
@@ -194,7 +194,7 @@ class MigrationExecutor(object):
                     fake = True
             if not fake:
                 # Alright, do it normally
-                with self.connection.schema_editor() as schema_editor:
+                with self.connection.schema_editor(atomic=migration.atomic) as schema_editor:
                     state = migration.apply(state, schema_editor)
         # For replacement migrations, record individual statuses
         if migration.replaces:
@@ -214,7 +214,7 @@ class MigrationExecutor(object):
         if self.progress_callback:
             self.progress_callback("unapply_start", migration, fake)
         if not fake:
-            with self.connection.schema_editor() as schema_editor:
+            with self.connection.schema_editor(atomic=migration.atomic) as schema_editor:
                 state = migration.unapply(state, schema_editor)
         # For replacement migrations, record individual statuses
         if migration.replaces:

django/db/migrations/migration.py

@@ -48,6 +48,10 @@ class Migration(object):
     # introspection. If False, never perform introspection.
     initial = None
 
+    # Whether to wrap the whole migration in a transaction. Only has an effect
+    # on database backends which support transactional DDL.
+    atomic = True
+
     def __init__(self, name, app_label):
         self.name = name
         self.app_label = app_label
@@ -114,8 +118,10 @@ class Migration(object):
             old_state = project_state.clone()
             operation.state_forwards(self.app_label, project_state)
             # Run the operation
-            if not schema_editor.connection.features.can_rollback_ddl and operation.atomic:
-                # We're forcing a transaction on a non-transactional-DDL backend
+            atomic_operation = operation.atomic or (self.atomic and operation.atomic is not False)
+            if not schema_editor.atomic_migration and atomic_operation:
+                # Force a transaction on a non-transactional-DDL backend or an
+                # atomic operation inside a non-atomic migration.
                 with atomic(schema_editor.connection.alias):
                     operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
             else:

django/db/migrations/operations/special.py

@@ -139,7 +139,7 @@ class RunPython(Operation):
 
     reduces_to_sql = False
 
-    def __init__(self, code, reverse_code=None, atomic=True, hints=None, elidable=False):
+    def __init__(self, code, reverse_code=None, atomic=None, hints=None, elidable=False):
         self.atomic = atomic
         # Forwards code
         if not callable(code):
@@ -161,7 +161,7 @@ class RunPython(Operation):
         }
         if self.reverse_code is not None:
             kwargs['reverse_code'] = self.reverse_code
-        if self.atomic is not True:
+        if self.atomic is not None:
             kwargs['atomic'] = self.atomic
         if self.hints:
             kwargs['hints'] = self.hints