> Does the function operate on a single model? Will it be run when you
> already have the instance of the model?
To add to Collin's comment, it doesn't even have to operate on a
single model. It simply has to be a property of the model
itself. Thus, it would make sense to have something like
def Invoice(Model):
customer = ForeignKey(Customer)
invoice_dt = DateField(...)
paid = BooleanField(default=False)
def total_cost(self):
tax_rate = self.customer.tax_rate
i = 0
for lineitem in self.line_items:
i += lineitem.total_cost(tax_rate)
def LineItem(Model):
invoice = ForeignKey(Invoice, related_name="line_items")
description = CharField(...)
qty = IntegerField(...)
cost = DecimalField(...)
taxable = BooleanField(...)
def total_cost(self, tax_rate):
if not self.taxable: tax_rate = 1
return self.qty * self.cost * tax_rate
Note that each model has a total_cost() method, which is clearly
associated with calculating the "total cost" of the model. In
the case of the Invoice.total_cost(), it references sub-fields
within it such as the reverse foreign key collection "line_items"
and the customer associated with the given Invoice.
I've also made methods based not on the "request" object (which
is generally a bad smell as Alex mentioned), but on the user
object contained within the request:
class MyModel(Model)
...
def allowed_objects(cls, user):
return MyModel.filter(fieldname__user__id = user.id)
allowed_objects = classmethod(allowed_objects)
so I can use
MyMode.allowed_objects(request.user).get(id=42)
where each model knows how to filter itself based on what a given
user can see.
-tim
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