id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
31803	Should ModelState.field_cache be documented?	Jaap Roes	nobody	"While writing a model method that relies on data of a related object I decided that it would be nice if this method could log a warning when it is called without the related object having been prefetched.

I came across this StackOverflow question https://stackoverflow.com/questions/36402129/is-there-a-way-to-check-whether-a-related-object-is-already-fetched and based on that I implemented something like the following:

{{{
def my_method(self):
    if 'related_object' not in self._state.fields_cache:
        warnings.warn(
            'Calling MyModel.my_method without prefetching the related object can inefficient '
            'Consider adding prefetch_related/select_related to the originating queryset.',
            category=RuntimeWarning,
            stacklevel=3
        )
    ...
}}}

This seems to work as intended.

Checking the docs I see that `Model._state` ''is'' documented (https://docs.djangoproject.com/en/3.0/ref/models/instances/#state), but the `fields_cache` attribute ''isn't''.

Can I rely on `fields_cache`? Should I consider it to be public but undocumented, or private with the possibility of disappearing without notice?"	Cleanup/optimization	closed	Documentation	3.0	Normal	wontfix			Unreviewed	0	0	0	0	0	0