How do I implement Foreign Keys across different databases in Django when facing limitations?

How to Implement Foreign Keys Across Different Databases in Django

In Django, utilizing foreign keys is crucial for establishing relationships between objects stored across multiple databases. However, this process can present a challenge when models for different databases are involved.

Understanding Cross-Database Foreign Key Limitations

Django currently prohibits foreign key relationships between models residing in separate databases. This restriction stems from the absence of support for cross-database relationships. If you employ a database router to partition models, foreign key relationships must be limited to within a single database.

The Error: Assigning Incompatible Models

The error encountered in your code stems from the attempt to assign a LinkModel instance from the "urls" database to a NewsModel instance from the default database. Since these models belong to different databases, Django raises an error preventing this assignment.

The Solution: Manual Foreign Key Assignment

To circumvent this limitation, you can implement a custom workaround. Instead of assigning the LinkModel instance directly through the NewsModel's link field, manually set the foreign key field in the database:

<code class="python">from django.db import connection

# Retrieve the LinkModel instance from the database
link = LinkModel.objects.using('urls').get(id=1)

# Execute a raw SQL query to update the foreign key value in the default database
with connection.cursor() as cursor:
    cursor.execute(
        "UPDATE news SET link_id = %s WHERE title = %s",
        [link.id, "test"]
    )</code>

This approach manually updates the foreign key value in the default database, effectively linking the news item to the desired LinkModel instance.

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