ManyToManyRel.get_related_field() doesn't account for defined through to_field

[Simon Charette]

While investigating edge cases to_field_allowed has to cope with I stumbled upon the following unusual scenario:

class Ingredient(models.Model):
    iname = models.CharField(max_length=20, unique=True)

class Recipe(models.Model):
    rname = models.CharField(max_length=20, unique=True)
    ingredients = models.ManyToManyField(
        Ingredient, through='RecipeIngredient', related_name='recipes'
    )

class RecipeIngredient(models.Model):
    ingredient = models.ForeignKey(Ingredient, to_field='iname')
    recipe = models.ForeignKey(Recipe, to_field='rname')

Which is not correctly handled by the actual ManyToManyRel.get_related_field() implementation that assumes ​this is always the primary key on the target model.

Since ManyToManyRel is an implementation detail the provided test is against ManyToManyField.get_choices() method which relies on it. The patch also includes tests for related managers interaction since this reference scenario was not tested in m2m_through yet.

The fact that the implementation doesn't actually return the correct field means that we don't have to backport the required to_field_allowed adjustments (which I'll submit as another ticket/patch to facilitate backports of #23754 and #23839) since it never worked in previous Django versions.

[Simon Charette]

Updated patch.

[Tim Graham <timograham@…>]

In c7087bc777a078f71a0705d9d0eba51d2679a206:

Fixed #23862 -- Made ManyToManyRel.get_related_field() respect to_field.

[Simon Charette <charette.s@…>]

In 3a9aa155e2f7326df669953980ac87e78e932c43:

Fixed #23915 -- Made sure m2m fields through non-pk to_field are allowed in the admin.

refs #23754, #23862