Clarify why related ManyToMany fields with a custom intermediary model disable the remove() method.

[Antoine Auberger]

It is currently impossible to add or create related item to a many-to-many relationship with custom intermediary model, with reason, as the custom model extra fields won't be populated.
​This is documented and explained.

However, it is also not possible to remove related items from the collection, and the only reason given in the doc is : "The remove() method is disabled for similar reasons".

I've try to find a justification for this though the code/docs/history but I couldn't and this seems unjustified for me.
All the informations needed to remove the relation are provided if you just use this method e.g. mysourceobject.relatedobjects.remove(targetobject).
I don't see why ​it is disabled, and therefore would like to ask for a design change here, or an explanation.

[Tim Graham]

Russ called out the remove() method in ticket:6095#comment:33 but I fail to understand that reasoning as well. There doesn't seem to be any problem modify the relevant tests (patch attached - would need doc updates too). Are we missing something?

[Baptiste Mispelon]

I had a chat with Russ and he raised the following point. Consider a custome through table that (unlike the auto-generated one) doesn't enforce unicity on the (model1, model2) pair (which is a valid usecase).

Something like this:

class Pizza(models.Model):
    name = models.CharField(max_length=50)


class Customer(models.Model):
    name = models.CharField(max_length=50)
    orders = models.ManyToManyField('Pizza', through='Order')


class Order(models.Model):
    pizza = models.ForeignKey('Pizza')
    customer = models.ForeignKey('Customer')
    ordered_on = models.DateTimeField(default=timezone.now)

Doing some_customer.orders.remove(some_pizza) doesn't give enough information as to which Order instance should actually be deleted.

It seems to me that we should just better document the reason behind this, considering it's not exactly obvious.

[Tim Graham]

Thanks Baptiste!

[Benjamin Phillips]

Took a stab at a documentation patch to clarify this issue.

[Benjamin Phillips]

Made a PR with for my patch (PR 5728 ​https://github.com/django/django/pull/5728)

[Tim Graham <timograham@…>]

In d508326:

Fixed #25672 -- Clarified why related ManyToManyFields with a custom intermediate model disable the remove() method.