Opened 6 years ago

Closed 2 years ago

Last modified 2 years ago

#14481 closed Cleanup/optimization (fixed)

better documentation of the default m2m "through" class

Reported by: Jonathan Morgan Owned by: nobody
Component: Documentation Version: master
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

It would be polite to document somewhere the exact characteristics of the default m2m "through" class. I looked today and could not find that anywhere in the documentation.

Specifically, I think you want something like this in the Model page where it talks about ManyToManyFields:

When you create a ManyToManyField relation without a "through" argument, there is still a "through" class and table created to hold the association.

The class is:

<model_that_declares_field>.<ManyToManyField_name>.through

This is a model instance for a table with three fields:

  • id
  • <containing_model>_id
  • <other_model>_id

This class can be used to query associated records for a given model instance, and can also be used in static methods (like m2m_changed handlers) to query the association table to pull in instances that are associated either with Model A or Model B so that you can process them. The through class for a given association is the class that raises the m2m_changed signal, and so it is the class you need to associate an m2m_changed signal handler with, not the Model that contains the association.

It is also dynamically generated when the Model class that declares the ManyToManyField relation is imported, so you can only access "through" classes associated with a given pair of models after importing the model that declares the ManyToManyField (and you won't be able to access it if you only import the model that is the "other" side of the relation).

Change History (8)

comment:1 Changed 6 years ago by Gabriel Hurley

Triage Stage: UnreviewedAccepted

Related to but separate from #13586. Let's make sure these two don't end up duplicating each other in content.

comment:2 Changed 6 years ago by Julien Phalip

Severity: Normal
Type: Cleanup/optimization

comment:3 Changed 5 years ago by Aymeric Augustin

UI/UX: unset

Change UI/UX from NULL to False.

comment:4 Changed 5 years ago by Aymeric Augustin

Easy pickings: unset

Change Easy pickings from NULL to False.

comment:5 Changed 2 years ago by jorgecarleitao

Has patch: set

comment:6 Changed 2 years ago by Tim Graham <timograham@…>

Resolution: fixed
Status: newclosed

In de9012907063e210e97c0d65cde312ff6174d420:

Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.

comment:7 Changed 2 years ago by Tim Graham <timograham@…>

In 807aadaaad317b67965052048b3a2fb92a1aaa79:

[1.7.x] Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.

Backport of de90129070 from master

comment:8 Changed 2 years ago by Tim Graham <timograham@…>

In 6b52e1793c682d4f9c8f030dcbe6908ddddf875e:

[1.6.x] Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.

Backport of de90129070 from master

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