I use django nonrel/djangoappengine on google app engine. It is not possible to specify composite primary keys directly. However it should be possible to emulate this behaviour. I am wondering about what is the best approach.
Problem
Conside the following datamodel
class AccumuatedSales(models.Model):
salesman = models.ForeignKey(Salesman)
year = models.Postiveinteger()
totalSales = models.PositiveInteger()
I want (salesman, year) to be treated as a primary key. That is, if I do
asby1 = AccumulatedSales(salesman='Joe', year=2010, totalSales=100)
asby1.save()
asby2 = AccumulatedSales(salesman='Joe', year=2010, totalSales=200)
asby2.save()
The 'table' AccumulatedSales should contain one row. So the second save overwrites the first.
Possible Solution
class AccumuatedSales(models.Model):
key = models.CharField(primary_key=True,, default=None, editable=False)
salesman = models.ForeignKey(Salesman)
year = models.Postiveinteger()
totalSales = models.PositiveInteger()
def save(self):
self.key = someHashFunction(self.salesman_id, self.year)
super(AccumulatedSales, self).save()
Questions
- is this approach good or "the right one"?
- What is the best datatype for the field key? Personally I would like to have som 128-bit field, but that is not available to my knowledge.
