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

[anonymous]

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.

[Russell Keith-Magee]

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.