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

Thanks Markus Holtermann and Tim Graham for the review.

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