Getting only the models related to a Django queryset

Question

I don't have much experience with Django (I'm using 1.3) so I have the feeling on the back of my head that this is a dumb question... But anyway:

I have models like this:

class User(models.Model):
    name = models.CharField()

class Product(models.Model):
    name = models.CharField()
    public = models.BooleanField()

class Order(models.Model):
    user = models.ForeignKey(User)
    product = models.ManyToManyField(Product, through='OrderProduct')

class OrderProduct(models.Model):
    product = models.ForeignKey(Product)
    order = models.ForeignKey(Order)
    expiration = models.DateField()

And let's say I do some query like this

Product.objects.filter(order__status='completed', order__user____id=2)

So I'd get all the products that User2 bought (let's say it's just Product1 ). Cool. But now I want the expiration for that product, but if I call Product1.orderproduct_set.all() I'm gonna get every entry of OrderProduct with Product1 , but I just want the one returned from my queryset. I know I can just run a different query on OrderProducts, but that would be another hit on the database just to bring back data the query I ran before can already get. .query on it gives me:

SELECT "shop_product"."id", "shop_product"."name"
FROM "shop_product"
INNER JOIN "shop_orderproducts" ON ("shop_product"."id" = "shop_orderproducts"."product_id")
INNER JOIN "shop_order" ON ("shop_orderproducts"."order_id" = "shop_order"."id")
WHERE ("shop_order"."user_id" = 2  AND "shop_order"."status" = completed )
ORDER BY "shop_product"."ordering" ASC

If I could SELECT * instead of specific fields I'd have all the data that I need in one query. Is there anyway to build that query and get only the data related to it?

EDIT I feel I need to clarify some points, I'm sorry I haven't been clearer:

1. I'm not querying against OrderProduct because some products are public and don't have to be bought but I still have to list them, and they'd not be returned by a query against OrderProduct

2. The result I'm expecting is a list of products, along with their Order data (in case they have it). In JSON, it'd look somewhat like this

   [{id: 1, order: 1, expiration: 2013-03-03, public: false}, {id: 1, order: , expiration: , public: true

Thanks

Answer 1

I'm gonna get every entry of OrderProduct with Product1, but I just want the one returned from my queryset.

You just want which "one"? Your query is filtering on the Product model, so all Users , Orders , and OrderProducts associated with each of the Products in the returned queryset will be accessible.

If you want one specific OrderProduct , then you should be filtering as op = OrderProduct.objects.filter(xxxxx) and then accessing the models up the chain like so:

op.product , op.order , etc.

Answer 2

I would have suggested the method prefetch_related , but this isn't available in Django 1.3.

Dan Hoerst is right about selecting from OrderProduct , but that still hits the database more than necessary. We can stop that by using the select_related method.

>>> from django.db import connection
>>> len(connection.queries)
0
>>> first_result = OrderProduct.objects.select_related("order__user", "product")
...               .filter( order__status="completed",
...                        order__user__pk=2 )[0]
>>> len(connection.queries)
1
>>> name = first_result.order.user.name
>>> len(connection.queries)
1
>>> product_name = first_result.product.name
>>> len(connection.queries)
1