title: How to use django models with foreign keys in different DBs?
body: <p>I have 2 models for 2 different databases:<br>
  Databases were created manually but it should change nothing.  </p>
  
  <pre><code>class LinkModel(models.Model): # in 'urls' database
      id = models.AutoField(primary_key=True)
      host_id = models.IntegerField()
      path = models.CharField(max_length=255)
  
      class Meta:
          db_table = 'links'
          app_label = 'testapp'
  
      def __unicode__(self):
          return self.path
  
  class NewsModel(models.Model):  # in default database
      id = models.AutoField(primary_key=True)
      title = models.CharField(max_length=50)
      link = models.ForeignKey(LinkModel)
  
      class Meta:
          db_table = 'news'
          app_label = 'test'
  
      def __unicode__(self):
          return self.title
  </code></pre>
  
  <p>After the following code an error raises</p>
  
  <pre><code>newsItem, created = NewsModel.objects.get_or_create( title="test" )
  link = LinkModel.objects.using('urls').get( id=1 )
  newsItem.link = link  # error!
  
   Cannot assign "&lt;LinkModel: /&gt;": instance is on database "default", value is on database "urls"
  </code></pre>
  
  <p>Why can't I use foreign key and a model for different database?</p>