#23862: ManyToManyRel.get_related_field() doesn't account for defined through
to_field
-------------------------------------+-------------------------------------
Reporter: charettes | Owner: charettes
Type: Bug | Status: new
Component: Database | Version: master
layer (models, ORM) | Keywords:
Severity: Normal | Has patch: 1
Triage Stage: | Needs tests: 0
Unreviewed | Easy pickings: 0
Needs documentation: 0 |
Patch needs improvement: 0 |
UI/UX: 0 |
-------------------------------------+-------------------------------------
While investigating edge cases `to_field_allowed` has to cope with I
stumbled upon the following unusual scenario:
{{{#!python
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
[https://github.com/django/django/blob/4b9eb7602d6d8756933d1a767004155a263cc150/django/db/models/fields/related.py#L1349-L1355
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/23862>
Django <https://code.djangoproject.com/>
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