[Django] #20835: Allow standard m2m behaviour on through-relationships if extra-fields are nullable or have default values

[Django]

#20835: Allow standard m2m behaviour on through-relationships if extra-fields are
nullable or have default values
----------------------------------------------+------------------------
Reporter: anonymous | Owner: nobody
Type: New feature | Status: new
Component: Database layer (models, ORM) | Version: 1.6-beta-1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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I'm going to illustrate this feature using example from the documentation
with minor changes:
https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-
many-to-many-relationships

{{{
class Person(models.Model):
name = models.CharField(max_length=128)

class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')

class Membership(models.Model):
person = models.ForeignKey(Person)
group = models.ForeignKey(Group)
date_joined = models.DateField(null=True) # added null=True
invite_reason = models.CharField(max_length=64, default='unknown') #
added default='unknown'
}}}

{{{
# THIS WILL NOT WORK
>>> beatles.members.add(john)
# NEITHER WILL THIS
>>> beatles.members.create(name="George Harrison")
# AND NEITHER WILL THIS
>>> beatles.members = [john, paul, ringo, george]
}}}

This is due to the fact that extra fields must be populated. It's okay.
But when extra-fields are nullable or have default values it's makes sense
to allow adding entries via .add() method. Extra fields can be populated
on the fly, using default values or nulls, if nulls are allowed.

So I propose, that in this case the following should work:
{{{
>>> beatles.members.add(john) # Creates a Membership with person=John,
date_joined=null, invite_reason='unknown'
}}}

I've looked up the code which is blocking this behaviour:
django/db/models/fields/related.py:564
{{{
if rel.through._meta.auto_created: # it's here
def add(self, *objs):
self._add_items(self.source_field_name,
self.target_field_name, *objs)

# If this is a symmetrical m2m relation to self, add the
mirror entry in the m2m table
if self.symmetrical:
self._add_items(self.target_field_name,
self.source_field_name, *objs)
add.alters_data = True
}}}

In some places there are also exceptions:
{{{
raise AttributeError("Cannot use create() on a ManyToManyField
which specifies an intermediary model. Use %s.%s's Manager instead." %
(opts.app_label, opts.object_name))
raise AttributeError("Cannot set values on a ManyToManyField
which specifies an intermediary model. Use %s.%s's Manager instead." %
(opts.app_label, opts.object_name))
}}}

So actually, if one comments out these "raises" and unlocks the .add()
method then everything works smoothly. Even the ModelAdmin can now save
m2m's without need to write your own save_m2m method. Of course, only if
all the extra fields are nullable or have default values.

--
Ticket URL: <https://code.djangoproject.com/ticket/20835>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#20835: Allow standard m2m behaviour on through-relationships if extra-fields are
nullable or have default values

-------------------------------------+-------------------------------------
Reporter: anonymous | Owner: nobody
Type: New feature | Status: closed
Component: Database layer | Version:
(models, ORM) | 1.6-beta-1
Severity: Normal | Resolution: duplicate
Keywords: | Triage Stage:
Has patch: 0 | Unreviewed
Needs tests: 0 | Needs documentation: 0
Easy pickings: 0 | Patch needs improvement: 0
| UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by russellm):

* status: new => closed
* needs_better_patch: => 0
* resolution: => duplicate
* needs_tests: => 0
* needs_docs: => 0

Comment:

Duplicate of [9475], and some others; the API implications of this were
considered as part of [6095], which was the ticket that added manual m2m
through models.

--
Ticket URL: <https://code.djangoproject.com/ticket/20835#comment:1>