Unique set of fields in a model

Hello,
I'm looking for a way to define uniquness for a set of more than one
field in a model.
For example:

class Item(models.Model):
order = models.ForeignKey(Order)
number = models.IntegerField()
...

I want the number to be unique per order, and not unique throughout
the table.
How do I define that the combination <order, number> should be unique?

Thanks,
Ilya

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