django Multiple db with read only

Could anyone be so kind to explain me what is meant in the documentation: https://docs.djangoproject.com/en/1.7/topics/db/multi-db/#topics-db-multi-db-hints

I've added a new db in myproject/settings.py

DATABASES = {
    'default': {
    'ENGINE': 'django.db.backends.postgresql_psycopg2',
    'NAME': 'testdb',
    'USER': 'test',
  },
'db2': {
    'ENGINE': 'mysql.connector.django',
    'NAME': 'db2a',
}   }

but this command work only if I replace default with db2:

python manage.py inspectdb > models.py

now that I get the models from db2, how can I read from them swithching back db2 and default?

1) if I try a makemigrations, it see all new models but then it try to crate them, and I need only to read them because they already exists...

Now we’ll need to handle routing. First we want a router that knows to send queries for the auth app to auth_db

2) In which file should be added the AuthRouter(object): ? I can add a file called routers.py next to settings.py?

 from django.conf import settings

class db_Router(object):
    def db_for_read(self, model, **hints):
        return 'db2'

    def db_for_write(self, model, **hints):
        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        db_list = ('default', 'db2')
        if obj1._state.db in db_list and obj2._state.db in db_list:
            return True
        return None

    def allow_migrate(self, db, model):
        return True

    def allow_syncdb(self, db, model):
        if db == 'db2':
            return model._meta.app_label == 'appofdb2'
        elif model._meta.app_label == 'appofdb2':
            return False
        return None

this should work? or I'm completely out of way?