How to Handle Foreign Key Relationships with Models in Different Databases in Django?

Foreign Key and Cross-Database Constraints

When working with Django models stored in different databases, a common issue arises with foreign keys. For example, consider the following:

class LinkModel(models.Model): # in 'urls' database</p>
<pre class="brush:php;toolbar:false">id = models.AutoField(primary_key=True)
host_id = models.IntegerField()
path = models.CharField(max_length=255)

class NewsModel(models.Model): # in default database

id = models.AutoField(primary_key=True)
title = models.CharField(max_length=50)
link = models.ForeignKey(LinkModel)

Upon attempting to assign a foreign key to a model from a different database, an error will be raised:

Cannot assign "<LinkModel: />": instance is on database "default", value is on database "urls"<br>

This occurs because Django does not currently support cross-database foreign key relationships. All foreign key relationships must be defined within a single database.

Overcoming the Limitation

To resolve this issue, you can either create a router to explicitly route your models to specific databases or consider using a relational database management system (RDBMS) that supports cross-database relationships. Currently, Django does not provide an out-of-the-box solution for such relationships.

Alternatively, you can try applying a patch to the ForeignKey() class to address this issue, as described in the linked ticket.

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