Documentation of field.db_type() wrong

[Anssi Kääriäinen]

The docs (at ​https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#django.db.models.Field.db_type) claim that the db_type method is only called on DB creation - however that is wrong, and has been wrong since 2008. The db_type method is called from WHERE clause construction, too.

[Matthew Schinckel]

I'm working on a documentation patch.

However, it's not completely obvious to me how this should be cached. I mean, we could store it in the django cache, but that is implying that the django cache has been correctly set up. Most people will have set up the cache (as per ​https://docs.djangoproject.com/en/dev/topics/cache/#setting-up-the-cache), but what will happen if these settings have not been configured, and we attempt to store something in the cache?

Am I correct in assuming that this is something that would belong in the low-level cache, as the memoize decorator would only cache per instance of the field class (which gets recreated lots)?

Also, I'm not completely sure how to describe where it's being used. It's in the Constraint.process method, which indicates it "Returns a tuple of data suitable for inclusion in a WhereNode instance". What does this mean in high-level ORM-speak, or should I just replace the whole paragraph with something that indicates "This may be executed while preparing a query, so should be cached" or equivalent? I think this may be better, as the field class author will not know how the field may be used in the end.

[ANUBHAV JOSHI]

Please see the pull request:
​https://github.com/django/django/pull/2094

[Tim Graham]

Don't forget to set the trac flags appropriate to put the ticket in the review queue. I've marked "Has patch" and also "Patch needs improvement" per my comments on the PR. Thanks!

[ANUBHAV JOSHI]

Changes to pull requests done.

[Tim Graham]

Looks good to me, marking as RFC so @akaariai can take a look before it gets committed.

[Anubhav Joshi <anubhav9042@…>]

In 1a9f13df59e83eb6e0c20f96d878826519eca62a:

Fixed #21478 -- Corrected docs for when Field.db_type() is called.