[1.7.x] Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.

Backport of bb39037fcb from master

django/db/migrations/writer.py

@@ -10,7 +10,7 @@ import sys
 import types
 
 from django.apps import apps
-from django.db import models
+from django.db import models, migrations
 from django.db.migrations.loader import MigrationLoader
 from django.utils import datetime_safe, six
 from django.utils.encoding import force_text
@@ -44,7 +44,15 @@ class OperationWriter(object):
         argspec = inspect.getargspec(self.operation.__init__)
         normalized_kwargs = inspect.getcallargs(self.operation.__init__, *args, **kwargs)
 
-        self.feed('migrations.%s(' % name)
+        # See if this operation is in django.db.migrations. If it is,
+        # We can just use the fact we already have that imported,
+        # otherwise, we need to add an import for the operation class.
+        if getattr(migrations, name, None) == self.operation.__class__:
+            self.feed('migrations.%s(' % name)
+        else:
+            imports.add('import %s' % (self.operation.__class__.__module__))
+            self.feed('%s.%s(' % (self.operation.__class__.__module__, name))
+
         self.indent()
         for arg_name in argspec.args[1:]:
             arg_value = normalized_kwargs[arg_name]

tests/custom_migration_operations/more_operations.py

@@ -0,0 +1,22 @@
+from django.db.migrations.operations.base import Operation
+
+
+class TestOperation(Operation):
+    def __init__(self):
+        pass
+
+    @property
+    def reversible(self):
+        return True
+
+    def state_forwards(self, app_label, state):
+        pass
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def state_backwards(self, app_label, state):
+        pass
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        pass

tests/custom_migration_operations/operations.py

@@ -0,0 +1,26 @@
+from django.db.migrations.operations.base import Operation
+
+
+class TestOperation(Operation):
+    def __init__(self):
+        pass
+
+    @property
+    def reversible(self):
+        return True
+
+    def state_forwards(self, app_label, state):
+        pass
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def state_backwards(self, app_label, state):
+        pass
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+
+class CreateModel(TestOperation):
+    pass

tests/migrations/test_writer.py

@@ -16,6 +16,9 @@ from django.utils.deconstruct import deconstructible
 from django.utils.translation import ugettext_lazy as _
 from django.utils.timezone import get_default_timezone
 
+import custom_migration_operations.operations
+import custom_migration_operations.more_operations
+
 
 class TestModel1(object):
     def upload_to(self):
@@ -222,3 +225,22 @@ class WriterTests(TestCase):
                 expected_path = os.path.join(base_dir, *(app.split('.') + ['migrations', '0001_initial.py']))
                 writer = MigrationWriter(migration)
                 self.assertEqual(writer.path, expected_path)
+
+    def test_custom_operation(self):
+        migration = type(str("Migration"), (migrations.Migration,), {
+            "operations": [
+                custom_migration_operations.operations.TestOperation(),
+                custom_migration_operations.operations.CreateModel(),
+                migrations.CreateModel("MyModel", (), {}, (models.Model,)),
+                custom_migration_operations.more_operations.TestOperation()
+            ],
+            "dependencies": []
+        })
+        writer = MigrationWriter(migration)
+        output = writer.as_string()
+        result = self.safe_exec(output)
+        self.assertIn("custom_migration_operations", result)
+        self.assertNotEqual(
+            result['custom_migration_operations'].operations.TestOperation,
+            result['custom_migration_operations'].more_operations.TestOperation
+        )