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

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

docs/howto/writing-migrations.txt

@@ -184,6 +184,53 @@ the respective field according to your needs.
   migration is running. Objects created after the ``AddField`` and before
   ``RunPython`` will have their original ``uuid``’s overwritten.
 
+.. _non-atomic-migrations:
+
+Non-atomic migrations
+~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 1.10
+
+On databases that support DDL transactions (SQLite and PostgreSQL), migrations
+will run inside a transaction by default. For use cases such as performing data
+migrations on large tables, you may want to prevent a migration from running in
+a transaction by setting the ``atomic`` attribute to ``False``::
+
+    from django.db import migrations
+
+    class Migration(migrations.Migration):
+        atomic = False
+
+Within such a migration, all operations are run without a transaction. It's
+possible to execute parts of the migration inside a transaction using
+:func:`~django.db.transaction.atomic()` or by passing ``atomic=True`` to
+``RunPython``.
+
+Here's an example of a non-atomic data migration that updates a large table in
+smaller batches::
+
+    import uuid
+
+    from django.db import migrations, transaction
+
+    def gen_uuid(apps, schema_editor):
+        MyModel = apps.get_model('myapp', 'MyModel')
+        while MyModel.objects.filter(uuid__isnull=True).exists():
+            with transaction.atomic():
+                for row in MyModel.objects.filter(uuid__isnull=True)[:1000]:
+                    row.uuid = uuid.uuid4()
+                    row.save()
+
+    class Migration(migrations.Migration):
+        atomic = False
+
+        operations = [
+            migrations.RunPython(gen_uuid),
+        ]
+
+The ``atomic`` attribute doesn't have an effect on databases that don't support
+DDL transactions (e.g. MySQL, Oracle).
+
 Controlling the order of migrations
 ===================================
 

docs/ref/migration-operations.txt

@@ -269,7 +269,7 @@ be removed (elided) when :ref:`squashing migrations <migration-squashing>`.
 ``RunPython``
 -------------
 
-.. class:: RunPython(code, reverse_code=None, atomic=True, hints=None, elidable=False)
+.. class:: RunPython(code, reverse_code=None, atomic=None, hints=None, elidable=False)
 
 Runs custom Python code in a historical context. ``code`` (and ``reverse_code``
 if supplied) should be callable objects that accept two arguments; the first is
@@ -354,16 +354,19 @@ the ``schema_editor`` provided on these backends; in this case, pass
 
 On databases that do support DDL transactions (SQLite and PostgreSQL),
 ``RunPython`` operations do not have any transactions automatically added
-besides the transactions created for each migration (the ``atomic`` parameter
-has no effect on these databases). Thus, on PostgreSQL, for example, you should
-avoid combining schema changes and ``RunPython`` operations in the same
-migration or you may hit errors like ``OperationalError: cannot ALTER TABLE
-"mytable" because it has pending trigger events``.
+besides the transactions created for each migration. Thus, on PostgreSQL, for
+example, you should avoid combining schema changes and ``RunPython`` operations
+in the same migration or you may hit errors like ``OperationalError: cannot
+ALTER TABLE "mytable" because it has pending trigger events``.
 
 If you have a different database and aren't sure if it supports DDL
 transactions, check the ``django.db.connection.features.can_rollback_ddl``
 attribute.
 
+If the ``RunPython`` operation is part of a :ref:`non-atomic migration
+<non-atomic-migrations>`, the operation will only be executed in a transaction
+if ``atomic=True`` is passed to the ``RunPython`` operation.
+
 .. warning::
 
     ``RunPython`` does not magically alter the connection of the models for you;
@@ -382,6 +385,11 @@ attribute.
 
     The ``elidable`` argument was added.
 
+.. versionchanged:: 1.10
+
+    The ``atomic`` argument default was changed to ``None``, indicating that
+    the atomicity is controlled by the ``atomic`` attribute of the migration.
+
 ``SeparateDatabaseAndState``
 ----------------------------
 

docs/releases/1.10.txt

@@ -258,6 +258,9 @@ Migrations
   :class:`~django.db.migrations.operations.RunPython` operations to allow them
   to be removed when squashing migrations.
 
+* Added support for :ref:`non-atomic migrations <non-atomic-migrations>` by
+  setting the ``atomic`` attribute on a ``Migration``.
+
 Models
 ~~~~~~