Django 1.7：我可以使用prefetch_related（）来获取不带有related_name的ForeignKey吗？

Question

I am trying to display Items to a page with the least amount of database hits as possible (there are a lot of Items). Suppose my models look like this:

Class Item(models.Model):
    name = models.CharField(max_length=50, unique=True)

Class ItemAttribute(models.Model):
    item = models.ForeignKey(Item)
    name = models.ForeignKey(ItemAttributeName)

Class ItemAttributeName(models.Model):
    name = models.CharField(max_length=50, unique=True)

Notice there is no related name. Items have a list of attributes, a good related_name I could use would be item_attributes. But for this question I am not doing that. I was wondering if there is a way to query a list of Items and their attributes so I can determine if the item is used or new.

Item.ItemAttribute.ItemAttributeName.name = "USED"

would (should) look like

Item.item_attributes[0].name.name = "USED"

Something like that, you get the gist.

Is it possible to query the attributes name used prefetch_related()? I mean I know _related is in the name so this may seem like a dumb question but I was wondering if it's possible. The obvious answer is to stop being an dummy and add a related_name but don't worry about that right now.

Answer 1

There's never any need to define a related_name (except if there is a conflict). Django defines a perfectly good default.

However, your model structure has deeper problems than that. ItemAttribute is just the through table in a many-to-many relationship. You don't need it at all, because Django defines it automatically. Just declare the actual relationship you care about, which is between Item and Attribute:

class Item(models.Model):
    name = models.CharField(max_length=50, unique=True)
    attributes = models.ManyToManyField('ItemAttributeName')

Now you can use prefetch_related directly to fetch everything:

items = Item.objects.all().prefetch_related('attributes')

Answer 2

One possible solution is to retrieve a query of ItemAttributes such that

attributes = ItemAttribute.objects.filter(item=<given_item>)

Then look at names of those attributes...

for attribute in attributes:
    if attribute.name.name == "USED":
        # do something
    else:
        # do something else

But this solution doesn't use prefetch_related because it implies a given item from a list of item's you've already queried...

items = Item.objects.filter(<some_filter>)
for item in items:
    check_if_used(item)