jpwatts
commented
14 years ago Thanks for pointing this out; I've never tried using a ManyToManyField as a collection. It definitely doesn't work and the error message isn't helpful, either.
I'm going to generalize and say that any time you want to use a ManyToManyField as a collection, what you really need is an intermediate model with a PositionField. In your example, position is really an attribute of the relationship between a Product and a Category, rather than of a Product itself.
class Category(models.Model):
...
class Product(models.Model):
categories = models.ManyToManyField(Category, through='ProductCategory')
class ProductCategory(models.Model):
product = models.ForeignKey(Product)
category = models.ForeignKey(Category)
position = PositionField(collection=('product', 'category'))If you can think of any cases where the generalization doesn't hold, I'd love to hear about them. Otherwise, I'll probably change PositionField to raise an exception if a ManyToManyField is provided in the collection argument. At least that way the error message will be related to the actual problem, rather than a Django implementation detail.
If I try to add a collection named with a manyToManyField (like a category for a Product), I get this error:
'Product' instance needs to have a primary key value before a many-to-many relationship can be used.
I've used this code in my model:
position= PositionField(_('position'), collection='categories')