[1.7.x] Fixed #24052 -- Doc'd how to write data migrations with model…

…s in multiple apps.

Backport of b089759 from master

docs/topics/migrations.txt

@@ -435,68 +435,31 @@ You can pass a second callable to
 want executed when migrating backwards. If this callable is omitted, migrating
 backwards will raise an exception.
 
-.. _data-migrations-and-multiple-databases:
+Accessing models from other apps
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Data migrations and multiple databases
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When writing a ``RunPython`` function that uses models from apps other than the
+one in which the migration is located, the migration's ``dependencies``
+attribute should include the latest migration of each app that is involved,
+otherwise you may get an error similar to: ``LookupError: No installed app
+with label 'myappname'`` when you try to retrieve the model in the ``RunPython``
+function using ``apps.get_model()``.
 
-When using multiple databases, you may need to figure out whether or not to
-run a migration against a particular database. For example, you may want to
-**only** run a migration on a particular database.
-
-In order to do that you can check the database connection's alias inside a
-``RunPython`` operation by looking at the ``schema_editor.connection.alias``
-attribute::
-
-    from django.db import migrations
-
-    def forwards(apps, schema_editor):
-        if not schema_editor.connection.alias == 'default':
-            return
-        # Your migration code goes here
-
-    class Migration(migrations.Migration):
-
-        dependencies = [
-            # Dependencies to other migrations
-        ]
-
-        operations = [
-            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:
-
-.. snippet::
-    :filename: myapp/dbrouters.py
-
-    class MyRouter(object):
-
-        def allow_migrate(self, db, model):
-            return db == 'default'
-
-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
+In the following example, we have a migration in ``app1`` which needs to use
+models in ``app2``. We aren't concerned with the details of ``move_m1`` other
+than the fact it will need to access models from both apps. Therefore we've
+added a dependency that specifies the last migration of ``app2``::
 
     class Migration(migrations.Migration):
 
         dependencies = [
-            # Dependencies to other migrations
+            ('app1', '0001_initial'),
+            # added dependency to enable using models from app2 in move_m1
+            ('app2', '0004_foobar'),
         ]
 
         operations = [
-            migrations.RunPython(forwards),
+            migrations.RunPython(move_m1),
         ]
 
 More advanced migrations