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
33208	Allow globally defining custom (i.e. with select_related) querysets for ModelChoiceFields/ForeignKeys	Matthijs Kooijman	nobody	"TL;DR: In cases where you have models with a foreignkey, a ModelChoiceField (e.g. using ModelAdmin), and a `__str__` method that uses a related field, a lot of queries might be duplicated. There does not currently seem to be a clean way to add select_related in the right place to fix this cleanly.

e.g. consider something like this:

{{{
from django.db import models
from django.contrib import admin

class Author(models.Model):
    name = models.CharField(max_length=255)

class Book(models.Model):
    name = models.CharField(max_length=255)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    def __str__(self):
        return ""{} by {}"".format(self.name, self.author.name)

class BookReview(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE)
    rating = models.IntegerField()

@admin.register(BookReview)
class BookReviewAdmin(admin.ModelAdmin):
    pass
}}}

When showing the admin change view for a Review, I get a ton of queries when the ModelChoiceField tries to render all options for the `book` field. For each Book in the database, it calls `__str__`, which does a single query to retrieve `self.author.name`.

The obvious fix for this would be to ensure that the ModelChoiceField queryset uses `select_related` to prefetch all data. I've found that this is possible by defining `formfield_for_foreignkey` on (in this case) `BookReviewAdmin`, e.g. something like:

{{{
    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == ""book"":
            kwargs['queryset'] = Book.objects.all().select_related('author')
        return super().formfield_for_foreignkey(db_field, request, **kwargs)
}}}

This particular implementation has the downside that it bypasses the default queryset processing of ModelAdmin (currently only applying the ordering for the ModelAdmin registered for Book, if any). It would be nicer to override `ModelAdmin.get_field_queryset`, but that does not seem to be a public API.

In any case, the bigger problem with this approach, is that it has to be applied for *every* ModelAdmin that has a ForeignKey to Book, which leads to duplication. Ideally, and that is the point of this issue, it would be possible to define this select_related business in the Book model or in BookAdmin, so it is used automatically.

I'm not quite sure *how* this would would work and where to put it, though. Conceptually, the select_related is needed because of how `__str__` is defined on the model, so defining this in the model would make sense. There is already the concept of different managers that can prepare querysets for different usecases, but I'm not sure if this case would warrant a new manager name. If it would, then when would this manager be used?

Conceptually I guess it's not even linked to the ForeignKey/ModelChoiceField itself, but it would be the ""manager to use when you want to call __str__"". Since using ModelChoiceField implies calling `__str__`, using this custom manager for ModelChoiceField (and/or from ForeignKey.dbfield) would also solve the problem, but might apply the  select_related also in cases where it is not really needed (expanding this further, you could of course just add the select_related to the default manager, which would work, but end up selecting too much in a lot of cases).

One pragmatic alternative could be to try and fix this at the admin level instead, where in this case BookAdmin could define something like `get_queryset_for_references()` or so, and have the `BookReviewAdmin.get_field_queryset()` use that (similar to how it already applies the ordering from the related field admin). Again, not sure how to exactly define this in an elegant way..."	New feature	closed	contrib.admin	3.2	Normal	wontfix			Unreviewed	0	0	0	0	0	0