django, access fields in intermediate model

Question

I'm creating a Person Group and Membership as described in django docs for intermediate model.

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

  def __unicode__(self):
    return self.name

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

  def __unicode__(self):
    return self.name

class Membership(models.Model):
  person = models.ForeignKey(Person)
  group = models.ForeignKey(Group)
  date_joined = models.DateField()
  invite_reason = models.CharField(max_length=64)

It is possible to access the Person from a Group object with:

>>>MyGroup.members.name

Does Django creates another query to fetch the Person? Can I access the date_joined field from a Group object?

The thing that confuses me is that I would expect to get the person name field with:

>>>MyGroup.members.person.name

What happens if a Person has a field 'name' and also the intermediate model have a field 'name'.

Answer 1

The members field in your example is a ManyToManyField, so it's a way to access many people rather than one person. The object that is under the members field is actually a special type of Manager, not a Person:

>>>print my_group.members
<django.db.models.fields.related.ManyRelatedManager object at 0x181f7d0>

To understand better what a Manager is, see the documentation.

To access a person's name you would do for example:

>>>for person in my_group.members.all():
>>>    print person.name

You cannot access the fields in your Membership model via the Manager in the members field. To access any of the fields in it you would do:

>>>for membership in my_group.membership_set.all():
>>>    print membership.date_joined

And so if you had a field called name in your Membership model, you would access it like this:

>>>for membership in my_group.membership_set.all():
>>>    print membership.name

A second way to access a Person's name would be:

>>>for membership in my_group.membership_set.all():
>>>    print membership.person.name

Hope that helps a little :)

Answer 2

Use the manager of the membership class:

MyGroup.membership_set.all()

instead of:

MyGroup.members.all()