On Mon, Mar 21, 2011 at 12:33:01AM -0700, Christophe Pettus wrote: > I'd like to make one more pitch for a slightly different > implementation here. My concern with CompositeField isn't based on > the fact that it doesn't map one-to-one with a field in the table; > it's that it doesn't have any of the semantics that are associated > with a field. In particular, it can't be: > > - Assigned to. > - Iterated over. > - Or even have a value. I disagree. The CompositeField would need to have a value to be able to implement a ForeignKey pointing to a modedel with a composite primary key.
The CompositeField itself would be just a proxy to the actual atomic
fields. You should be able to assign a tuple (or namedtuple) to it,
specifying the actual values for the fields. Similarly, you'll be able
to retrieve its value which is a tuple or a namedtuple.
This way, the following code would work for composite primary keys the
same way it works for simple keys:
class CompositeModel(models.Model):
a = models.IntegerField()
b = models.IntegerField()
key = models.CompositeField((a, b), primary_key=True)
class ReferencingModel(models.Model):
cm = models.ForeignKey(CompositeModel)
cminstance = CompositeModel.objects.get(something)
newref = ReferencingModel()
newref.cm = cminstance.pk
> My suggestion is to create an Index type that can be included in a
> class just like a field can. The example we've been using would
> then look like:
>
> class Foo(Model):
> x = models.FloatField()
> y = models.FloatField()
> a = models.ForeignKey(A)
> b = models.ForeignKey(B)
>
> coords = models.CompositeIndex((x, y))
> pair = models.CompositeIndex((a, b), primary_key=True)
>
> We could have FieldIndex (the equivalent of the current
> db_index=True), CompositeIndex, and RawIndex, for things like
> expression indexes and other things that can be specified just as a
> raw SQL string.
>
> I think this is a much better contract to offer in the API than one
> based on field which would have to throw exceptions left and right
> for most of the common field operations.
I don't see how ForeignKeys would be possible this way.
Michal Petrucha
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