get_or_create django model with ManyToMany field

Question

Suppose I have three django models:

class Section(models.Model):
    name = models.CharField()

class Size(models.Model):
    section = models.ForeignKey(Section)
    size = models.IntegerField()

class Obj(models.Model):
    name = models.CharField()
    sizes = models.ManyToManyField(Size)

I would like to import a large amount of Obj data where many of the sizes fields will be identical. However, since Obj has a ManyToMany field, I can't just test for existence like I normally would. I would like to be able to do something like this:

try:
    x = Obj(name='foo')
    x.sizes.add(sizemodel1) # these can be looked up with get_or_create
    ...
    x.sizes.add(sizemodelN) # these can be looked up with get_or_create
    # Now test whether x already exists, so I don't add a duplicate
    try:
        Obj.objects.get(x)
    except Obj.DoesNotExist:
        x.save()

However, I'm not aware of a way to get an object this way, you have to just pass in keyword parameters, which don't work for ManyToManyFields.

Is there any good way I can do this? The only idea I've had is to build up a set of Q objects to pass to get:

myq = myq & Q(sizes__id=sizemodelN.id)

But I am not sure this will even work...

Answer 1

Use a through model and then .get() against that.

http://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships

Once you have a through model, you can .get() or .filter() or .exists() to determine the existence of an object that you might otherwise want to create. Note that .get() is really intended for columns where unique is enforced by the DB - you might have better performance with .exists() for your purposes.

If this is too radical or inconvenient a solution, you can also just grab the ManyRelatedManager and iterate through to determine if the object exists:

object_sizes = obj.sizes.all()
exists = object_sizes.filter(id__in = some_bunch_of_size_object_ids_you_are_curious_about).exists()
if not exists: 
    (your creation code here)