Fixed #22583 -- Allowed RunPython and RunSQL to provide hints to the db router.

Thanks Markus Holtermann and Tim Graham for the review.

docs/ref/migration-operations.txt

@@ -206,7 +206,7 @@ Special Operations
 RunSQL
 ------
 
-.. class:: RunSQL(sql, reverse_sql=None, state_operations=None)
+.. class:: RunSQL(sql, reverse_sql=None, state_operations=None, hints=None)
 
 Allows running of arbitrary SQL on the database - useful for more advanced
 features of database backends that Django doesn't support directly, like
@@ -235,6 +235,11 @@ operation here so that the autodetector still has an up-to-date state of the
 model (otherwise, when you next run ``makemigrations``, it won't see any
 operation that adds that field and so will try to run it again).
 
+The optional ``hints`` argument will be passed as ``**hints`` to the
+:meth:`allow_migrate` method of database routers to assist them in making
+routing decisions. See :ref:`topics-db-multi-db-hints` for more details on
+database hints.
+
 .. versionchanged:: 1.7.1
 
     If you want to include literal percent signs in a query without parameters
@@ -245,6 +250,8 @@ operation that adds that field and so will try to run it again).
     The ability to pass parameters to the ``sql`` and ``reverse_sql`` queries
     was added.
 
+    The ``hints`` argument was added.
+
 .. attribute:: RunSQL.noop
 
     .. versionadded:: 1.8
@@ -258,7 +265,7 @@ operation that adds that field and so will try to run it again).
 RunPython
 ---------
 
-.. class:: RunPython(code, reverse_code=None, atomic=True)
+.. class:: RunPython(code, reverse_code=None, atomic=True, hints=None)
 
 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
@@ -267,6 +274,15 @@ match the operation's place in the project history, and the second is an
 instance of :class:`SchemaEditor
 <django.db.backends.schema.BaseDatabaseSchemaEditor>`.
 
+The optional ``hints`` argument will be passed as ``**hints`` to the
+:meth:`allow_migrate` method of database routers to assist them in making a
+routing decision. See :ref:`topics-db-multi-db-hints` for more details on
+database hints.
+
+.. versionadded:: 1.8
+
+    The ``hints`` argument was added.
+
 You are advised to write the code as a separate function above the ``Migration``
 class in the migration file, and just pass it to ``RunPython``. Here's an
 example of using ``RunPython`` to create some initial objects on a ``Country``

docs/releases/1.8.txt

@@ -462,6 +462,12 @@ Migrations
   attribute/method were added to ease in making ``RunPython`` and ``RunSQL``
   operations reversible.
 
+* The :class:`~django.db.migrations.operations.RunPython` and
+  :class:`~django.db.migrations.operations.RunSQL` operations now accept a
+  ``hints`` parameter that will be passed to :meth:`allow_migrate`. To take
+  advantage of this feature you must ensure that the ``allow_migrate()`` method
+  of all your routers accept ``**hints``.
+
 Models
 ^^^^^^
 
@@ -1029,6 +1035,14 @@ Miscellaneous
 * :func:`django.utils.translation.get_language()` now returns ``None`` instead
   of :setting:`LANGUAGE_CODE` when translations are temporarily deactivated.
 
+* The migration operations :class:`~django.db.migrations.operations.RunPython`
+  and :class:`~django.db.migrations.operations.RunSQL` now call the
+  :meth:`allow_migrate` method of database routers. In these cases the
+  ``model`` argument of ``allow_migrate()`` is set to ``None``, so the router
+  must properly handle this value. This is most useful when used together with
+  the newly introduced ``hints`` parameter for these operations, but it can
+  also be used to disable migrations from running on a particular database.
+
 .. _deprecated-features-1.8:
 
 Features deprecated in 1.8

docs/topics/db/multi-db.txt

@@ -150,7 +150,7 @@ A database Router is a class that provides up to four methods:
     used by foreign key and many to many operations to determine if a
     relation should be allowed between two objects.
 
-.. method:: allow_migrate(db, model)
+.. method:: allow_migrate(db, model, **hints)
 
     Determine if the ``model`` should have tables/indexes created in the
     database with alias ``db``. Return True if the model should be
@@ -293,7 +293,7 @@ send queries for the ``auth`` app to ``auth_db``::
                return True
             return None
 
-        def allow_migrate(self, db, model):
+        def allow_migrate(self, db, model, **hints):
             """
             Make sure the auth app only appears in the 'auth_db'
             database.
@@ -333,7 +333,7 @@ from::
                 return True
             return None
 
-        def allow_migrate(self, db, model):
+        def allow_migrate(self, db, model, **hints):
             """
             All non-auth models end up in this pool.
             """

docs/topics/migrations.txt

@@ -545,28 +545,26 @@ attribute::
             migrations.RunPython(forwards),
         ]
 
-You can also use your database router's ``allow_migrate()`` method, but keep in
-mind that the imported router needs to stay around as long as it is referenced
-inside a migration:
+.. versionadded:: 1.8
+
+You can also provide hints that will be passed to the :meth:`allow_migrate()`
+method of database routers as ``**hints``:
 
 .. snippet::
     :filename: myapp/dbrouters.py
 
     class MyRouter(object):
 
-        def allow_migrate(self, db, model):
-            return db == 'default'
+        def allow_migrate(self, db, model, **hints):
+            if 'target_db' in hints:
+                return db == hints['target_db']
+            return True
 
 Then, to leverage this in your migrations, do the following::
 
     from django.db import migrations
 
-    from myappname.dbrouters import MyRouter
-
     def forwards(apps, schema_editor):
-        MyModel = apps.get_model("myappname", "MyModel")
-        if not MyRouter().allow_migrate(schema_editor.connection.alias, MyModel):
-            return
         # Your migration code goes here
 
     class Migration(migrations.Migration):
@@ -576,7 +574,7 @@ Then, to leverage this in your migrations, do the following::
         ]
 
         operations = [
-            migrations.RunPython(forwards),
+            migrations.RunPython(forwards, hints={'target_db': 'default'}),
         ]
 
 More advanced migrations