Clarify ManyToManyField.through note about reverse accessors names for non-symmetrical recursive relationships.

[Fabio Sangiovanni]

Hello and happy new year everyone!

A note in the current ManyToManyField.through docs (​https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ManyToManyField.through) suggests that:

Recursive relationships using an intermediary model and defined as symmetrical (that is, with symmetrical=True, which is the default) can’t determine the reverse accessors names, as they would be the same.

But the value of the symmetrical argument seems irrelevant in this regard, since in both cases (True or False value) the system checks framework raises the same error (fields.E304).

Repro:

from django.db import models


class Relationship(models.Model):
    person1 = models.ForeignKey('Person', on_delete=models.CASCADE)
    person2 = models.ForeignKey('Person', on_delete=models.CASCADE)


class Person(models.Model):
    other = models.ManyToManyField('self', through=Relationship, symmetrical=False)

manage.py makemigrations yields:

SystemCheckError: System check identified some issues:

ERRORS:
app.Relationship.person1: (fields.E304) Reverse accessor for 'Relationship.person1' clashes with reverse accessor for 'Relationship.person2'.
	HINT: Add or change a related_name argument to the definition for 'Relationship.person1' or 'Relationship.person2'.
app.Relationship.person2: (fields.E304) Reverse accessor for 'Relationship.person2' clashes with reverse accessor for 'Relationship.person1'.
	HINT: Add or change a related_name argument to the definition for 'Relationship.person2' or 'Relationship.person1'.

for both values of the symmetrical argument of ManyToManyField.

Am I missing something?

Thanks!

[Mariusz Felisiak]

Good catch. IMO we can chop: "and defined as symmetrical (that is, with symmetrical=True, which is the default)". Would you like to prepare a patch?

[Fabio Sangiovanni]

Sure, I'm happy to open a PR for this. Will do as soon as I can.

[Fabio Sangiovanni]

PR ​https://github.com/django/django/pull/13838

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2d6c9b9:

Fixed #32310 -- Fixed note about reverse accessors for intermediate table for self-referential ManyToManyField.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 6d3a623:

[3.1.x] Fixed #32310 -- Fixed note about reverse accessors for intermediate table for self-referential ManyToManyField.

Backport of 2d6c9b97bc706aab1975f57e814461e90e389bb0 from master