[2.2.x] Fixed #27164 -- Fixed an example of using routers in multiple…

… databases docs.

Make sure that AuthRouter includes ContentType in the same database.

Backport of 608e060 from master

docs/topics/db/multi-db.txt

@@ -301,44 +301,51 @@ databases::
     }
 
 Now we'll need to handle routing. First we want a router that knows to
-send queries for the ``auth`` app to ``auth_db``::
+send queries for the ``auth`` and ``contenttypes`` apps to ``auth_db``
+(``auth`` models are linked to ``ContentType``, so they must be stored in the
+same database)::
 
     class AuthRouter:
         """
         A router to control all database operations on models in the
-        auth application.
+        auth and contenttypes applications.
         """
+        route_app_labels = {'auth', 'contenttypes'}
+
         def db_for_read(self, model, **hints):
             """
-            Attempts to read auth models go to auth_db.
+            Attempts to read auth and contenttypes models go to auth_db.
             """
-            if model._meta.app_label == 'auth':
+            if model._meta.app_label in self.route_app_labels:
                 return 'auth_db'
             return None
 
         def db_for_write(self, model, **hints):
             """
-            Attempts to write auth models go to auth_db.
+            Attempts to write auth and contenttypes models go to auth_db.
             """
-            if model._meta.app_label == 'auth':
+            if model._meta.app_label in self.route_app_labels:
                 return 'auth_db'
             return None
 
         def allow_relation(self, obj1, obj2, **hints):
             """
-            Allow relations if a model in the auth app is involved.
+            Allow relations if a model in the auth or contenttypes apps is
+            involved.
             """
-            if obj1._meta.app_label == 'auth' or \
-               obj2._meta.app_label == 'auth':
+            if (
+                obj1._meta.app_label in self.route_app_labels or
+                obj2._meta.app_label in self.route_app_labels
+            ):
                return True
             return None
 
         def allow_migrate(self, db, app_label, model_name=None, **hints):
             """
-            Make sure the auth app only appears in the 'auth_db'
-            database.
+            Make sure the auth and contenttypes apps only appear in the
+            'auth_db' database.
             """
-            if app_label == 'auth':
+            if app_label in self.route_app_labels:
                 return db == 'auth_db'
             return None