如何在接收模型的末端明确显示Django ManyToMany关系

Question

Relationships, particularly ManyToMany , in Django have always bothered me somewhat. In particular, since the relationship is only defined in one of the models, you can't tell from looking at the paired model what other relationships it might be hiding.

For example, from the Django Documentation :

class Topping(models.Model):
    # ...

class Pizza(models.Model):
    # ...
    toppings = models.ManyToManyField(Topping)

You can tell from looking at the code that I'd find out the relevant toppings for a pizza at Pizza.toppings . But you cannot tell that I would be able to tell what pizzas have a topping at Topping.Pizza_set --you have to look at the Pizza class to see this.

As a result, by looking at Toppings , I don't actually know the full range of fields that it has.

Is there any way around this or to make it more explicit? Or is there something that I'm missing?

Answer 1

This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:

class Topping(models.Model):
    # ...

class Pizza(models.Model):
    # ...

class PizzaToppings(models.Model):
    # '+' disables the reverse relationship
    pizza = models.ForeignKey(Pizza, related_name='+') 
    topping = models.ForeignKey(Topping, related_name='+')

... but of course then you'd lose some of the convenience of the ORM.

Answer 2

Found a way on Django's forum (lost link, sorry)

class Topping(models.Model):
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)

class Pizza(models.Model):
    toppings = models.ManyToManyField(Topping)