How to retrieve a polymorphic model from a single table in django - or how to implement polymorphic behaviour in django

Can I read polymorphic models from a single database table, with their behaviour depending on a (boolean) field of the model?

In one of my models the behaviour is slightly different if the instance is 'forward' vs. 'backward' or 'left' vs. 'right'. That leads to a lot of if-clauses and code duplication. So I want to have a Forward- and a Backward-variant of the model that encapsulate the different behaviours.

But how can I make the models manager return the instances of the right classes? Do I have to overwrite __init__ of the model?

Maybe it's easier to explain with an example. What I'm doing:

class Foo(models.Model):
    forward = models.BooleanField()
    other_fields = ...

    def do_foobar(bar):
        if self.forward:
            gap = bar.end_pos - bar.current_pos
            self.do_forward_move(max = gap)
            if self.pos==bar.end_pos:
                and so on ...
        else:
            gap = bar.current_pos - bar.start_pos
            self.do_backward_move(max = gap)
            if self.pos==bar.start_pos:
                and so on ...

What I want to do:

class Foo(models.Model):
    forward = models.BooleanField开发者_运维知识库()
    other_fields = ...

    def __init__(*args, **kwargs):
        """ return ForwardFoo or BackwardFoo 
        depending on the value of 'forward'"""
        How?

    def do_foobar(bar):
        gap = self.calculate_gap(bar)
        self.do_move(max = gap)
        if self.end_point_reached():
            and so on ...

class ForwardFoo(Foo):
    def calculate_gap(bar):
        return bar.end_pos - bar.current_pos
and so on ...

for f in Foo.objects.all():
    f.do_foobar(bar)

Or is there a totally different way to avoid this kind of code duplication?

---

Proxy models:

class Foo(models.Model):
    # all model attributes here

class ForwardFooManager(models.Manager):
    def get_query_set(self, *args, **kwargs):
        qs = super(ForwardFooManager, self).get_query_set(*args, **kwargs)
        return qs.filter(forward=True)

class ForwardFoo(Foo):
    class Meta:
        proxy = True

    objects = ForwardsFooManager()

    # methods for forward model

class BackwardFooManager(models.Manager):
    def get_query_set(self, *args, **kwargs):
        qs = super(BackwardFooManager, self).get_query_set(*args, **kwargs)
        return qs.filter(forward=False)

class BackwardFoo(Foo):
    class Meta:
        proxy = True

    objects = BackwardFooManager()

    # methods for backward model

The above creates two proxy models: one for forward, one for backward. Proxy models do not have their own database table; they use the same database table as the model they inherit from. (This also means you cannot add any additional fields to the proxy model, only methods.)

There's also a custom manager for to force each one to only return the subset of items that belong to each. Just add whatever specific methods you need and you're done.