Multi-Model Inheritance + Fixtures = Fixed

I just thought I’d take a moment to give everyone a heads up thatMulti-Model Inheritance fixture dumping and loading is now fixed, as per Russ Magee’s changeset 7600. This fixes a subtle problem that some of you may have seen in the form of foreign keys pointing to the wrong objects after using loaddata to restore an app.

The “pk” fields (usually ‘id’) in the serialized dump weren’t being loaded correctly, meaning that the objects id fields defaulted to the table’s incremental numbering rather than using the number specified in your object’s fixture’s “pk” field. This would not be an issue unless you deleted an object in your original table and thus had a gap in your primary keys (IE: they were no longer un-interrupted and sequential).

Let’s say you have three objects, 1 (Apple), 2 (Banana), 3 (Orange). You delete Banana (2) and now have entries with keys of 1 and 3 (non-sequential). Some other arbitrary models point at 1 and 3, and you now dumpdata your Fruit app and re-load it. The loader ignores the fact that Orange’s PK is 3 and instead just adds it right behind apple as PK 2 (sequential, the default behavior for primary keys when no value is specified). All of your other relations pointing at Orange in other tables are now broken, and you’re likely to see Exceptions thrown or models pointing to the wrong fruit (if you had more fruit defined to be compacted into PK 3).

I have a feeling I didn’t explain this well. In the event that you’d like further elaboration, see the Django-Users post where it’s outlined a little beter. For those using Multi-Model inheritance and fixtures, this was a definite show-stopper.

A big thinks goes out to Russ Magee for fixing this!