Can Django Foreign Keys Span Different Databases?

Cross-Database Limitations with Django Foreign Keys

In Django, while it's possible to use different databases for various models, cross-database relationships are restricted. This means that foreign key relationships cannot span different databases when using a router to partition models.

Consider the example provided:

<code class="python">class LinkModel(models.Model): # in 'urls' database
    ...

class NewsModel(models.Model):  # in default database
    ...
    link = models.ForeignKey(LinkModel)
    ...</code>

In this scenario, an error will arise when attempting to assign a LinkModel instance from the urls database to the NewsModel's link field:

<code class="python">newsItem, created = NewsModel.objects.get_or_create( title="test" )
link = LinkModel.objects.using('urls').get( id=1 )
newsItem.link = link  # error!</code>

Django Limitations

Django's current limitations prevent cross-database foreign key and many-to-many relationships. This is because any such relationships must be defined within a single database when using database partitioning.

Troubleshoot and Solution

There's a known issue with the Django ForeignKey() class, specifically in its validate() method. This issue exists in versions 1.2, 1.3, and 1.4rc1.

A patch is available to resolve this issue. Applying the patch should устранить the error and allow you to assign LinkModel instances from different databases to NewsModel's link field.

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