On Thu, 2009-10-22 at 07:57 -0700, Monika Sulik wrote:
> Hi,
>
> I was wondering what exactly are the advantages of having code like
> this:
>
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> class Foo (models.Model):
> bar_m2m = models.ManyToManyField(Bar,through='Foo_Bar')
>
> class Bar (models.Model):
> pass
>
> class Foo_Bar (models.Model):
> foo = models.ForeignKey(Foo)
> bar = models.ForeignKey(Bar)
> x = models.IntegerField()
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>
> over having the same code, but having the Foo class like so:
>
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> class Foo (models.Model):
> pass
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>
> In both cases I can use foo.foo_bar_set and bar.foo_bar_set (assuming
> foo is a Foo Object and bar is a Bar Object). Why would I need the
> bar_m2m variable?
> I'm probably missing something pretty obvious, but if someone could
> point out the answer to me that would be great :)
>
> Monika
>
With a many to many with a through table, you can either go directly to
the other end of the relationship, or look at the through table. Without
it, you cannot look directly at the relationship, only at the link.
Eg, Users can be in many UserGroups and UserGroups can have many Users.
If I use a m2m relationship with a through table, I can do both these
things:
class UserGroup(models.Model):
users=ManyToManyField(User, through='UserUserGroupPreferences')
class UserUserGroupPreferences(models.Model):
user=models.ForeignKey('User')
usergroup=models.ForeignKey('UserGroup')
is_group_super_user=models.BooleanField(default=False)
u=User.objects.get(...)
# I can see directly which groups a user is in
u.usergroup_set.all()
# I can also examine things about a particular user -> group association
u.userusergrouppreferences_set.filter(is_group_super_user=True)
Without the m2m field, I wouldn't be able to go directly to the
usergroup from the user, I would only be able to go via the through
table.
Cheers
Tom
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