Selecting distinct nested relation in Django

To describe the system quickly, I have a list of Orders. Each Order can have 1 to n Items associated with it. Each Item has a list of ItemSizes. Given the following models, which have been abbreviated in terms开发者_StackOverflow of fields for this question, my goal is to get a distinct list of ItemSize objects for a given Order object.

class ItemSize(models.Model):
    name = models.CharField(max_length=10, choices=SIZE_CHOICES)

class Item(models.Model):
    name = models.CharField(max_length=100)
    sizes = models.ManyToManyField(ItemSize)

class OrderItem(models.Model):
    order = models.ForeignKey(Order)
    item = models.ForeignKey(Item)

class Order(models.Model):
    some_field = models.CharField(max_length=100, unique=True)  

So... if I have:

o = Order.objects.get(id=1)
#how do I use the ORM to do this complex query? 
#i need o.orderitem_set.items.sizes (pseudo-code)

In your current set up, the answer by @radious is correct. However, OrderItems really shouldn't exist. Orders should have a direct M2M relationship with Items. An intermediary table will be created much like OrderItems to achieve the relationship, but with an M2M you get much simpler and more logical relations

class Order(models.Model):
    some_field = models.CharField(max_length=100, unique=True)
    items = models.ManyToManyField(Items, related_name='orders')

You can then do: Order.items.all() and Item.orders.all(). The query you need for this issue would be simplified to:

ItemSize.objects.filter(item__orders=some_order)

If you need additional data on the Order-Item relationship, you can keep OrderItem, but use it as a through table like:

class Order(models.Model):
    some_field = models.CharField(max_length=100, unique=True)
    items = models.ManyToManyField(Items, related_name='orders', through=OrderItem)

And you still get your simpler relationships.

ItemSize.objects.filter(items__orderitems__order=some_order)

Assuming you have reverse keys like:

• ItemSize.items - reverse fk for all items with such size
• Item.orderitems - reverse for all orderitems connected to item
• Item.orders - you can guess ;)

(AFAIR that names would be choose by default, but I'm not sure, you have to test it)

More informations about reverse key queries are available in documentation.