id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
20835	Allow standard m2m behaviour on through-relationships if extra-fields are nullable or have default values	anonymous	nobody	"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."	New feature	closed	Database layer (models, ORM)	1.6-beta-1	Normal	duplicate			Unreviewed	0	0	0	0	0	0