Cross-Database Foreign Key Limitations in Django
Django users may encounter an error when accessing foreign keys across different databases. This error occurs when a model in one database attempts to reference a model from another database.
Database Configuration
In the example provided, a LinkModel is defined in the 'urls' database, while a NewsModel is defined in the default database.
Foreign Key Issue
The error arises when attempting to assign a LinkModel instance to the foreign key field of a NewsModel object. The error message indicates that the assigned instance is on the 'urls' database while the NewsModel object is on the 'default' database.
Django Limitations
According to Django documentation, cross-database relationships are not supported for foreign keys and many-to-many relationships. This limitation is imposed due to compatibility issues and potential data integrity problems.
Trouble
In addition to the documented limitation, a bug in the ForeignKey class in Django versions 1.2, 1.3, and 1.4rc1 can also cause this issue.
Solution
One possible solution is to apply a patch that resolves the bug in the ForeignKey class. This patch can be found in the Django ticket system.
By applying this patch, it may be possible to establish foreign key relationships across different databases. However, users should be aware that this is not an officially supported feature and may introduce other potential issues.
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