How to use DB router in Django 1.4

问题:

I've been trying to setup Django 1.4.3 to use multiple DBs, but for the life of me I can't make it to work. I read the documentation and posts on SO, and did the following:

1) Add a second DB configuration in settings.py, as follows:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': '/tmp/django.db',
        'USER': '',
        'PASSWORD': '',
        'HOST': '',
        'PORT': '',
    },
    'db1' : {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': 'db1',
        'USER': 'fake',
        'PASSWORD': 'fake',
        'HOST': 'fake.host.com',
        'PORT': '3306',
    },
}

2) Create a routers.py and define a DB router as follows
(Note: According to SO posts, if you define a DB router in models.py, the router will not work)

class DBRouter(object):
    def db_for_read(self, model, **hints):
        return 'db1'

    def db_for_write(self, model, **hints):
        return 'db1'

    def allow_syncdb(self, db, model):
        return 'db1'

3) Add the following lines to settings.py
(Note: According to SO posts, these lines must be AFTER the DATABASES configurations

from django.db import connections
DATABASE_ROUTERS = ['fakeproject.routers.DBRouter',]

This was wrong. Do NOT put from django.db import connections here as it prevents the router to be registered

My symptoms:
Apparently, all my calls are routed through the default DB. Details below:

• Both DB settings work (I can perform manage.py indpectdb --database db1 successfully

• DATABASE_ROUTERS setting does not generate any complains (even if I put a wrong path to the DB router, or even a non-string object)

• When I attempt to access my objects via manage.py shell, I can do MyModel.objects.all() but when I actually try to iterate it, I'm told no such table. The default DB does not have that table, but 'db1' clearly has it as I generated the model by using inspectdb on it. As a proof, if I swap the DB configuration between db1 and default, I can access the object without problems.

Any help highly appreciated!

回答1:

I found out that the statement in step 3 "from django.db import connections " was preventing the DB router to be registered. When I removed this statement, the router was registered and stuff started to work as expected.

回答2:

I think the problem is probably arising because your routers.py only returns a reference to 'db1' but as you say you're only being routed to 'default' I'm unsure (I would expect it to be the only routed to 'db1'.

In routers.py create a master router class, and then subclass for each DB - and initialise with an app_label string so you can set them apart.

class MasterRouter(object):
    def __init__(self, app_label):
        super(MasterRouter, self).__init__()
        self.app_label = app_label

    def db_for_read(self, model, **hints):
        if model._meta.app_label == self.app_label:
            return self.app_label
        return None

    def db_for_write(self, model, **hints):
        if model._meta.app_label == self.app_label:
            return self.app_label
        return None

    def allow_relation(self, obj1, obj2, **hints):
        if obj1._meta.app_label == self.app_label or obj2._meta.app_label == self.app_label:
            return True
        return None

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

class DefaultRouter(MasterRouter):
    def __init__(self):
        super(DefaultRouter, self).__init__('default')

class DB1Router(MasterRouter):
    def __init__(self):
        super(DB1Router, self).__init__('db1')

and then in your settings.py declare the routers

DATABASE_ROUTERS = [ 'routers.DefaultRouter', 'routers.DB1Router' ]

ofcourse, you'll probably want to set up your MasterRouter class overrides differently.