如何在 Django 中创建查询集？

Question

item_list = Item.objects. filter(location_id=location_id)

image_list = Image.objects.filter(?????)

class Image(models.Model):
    item = models.ForeignKey(Item, models.on_delete=CASCADE)

I want to get only those images which item_id in the first queryset item_list

Answer 1

You can make use of the __in lookup [Django-doc] :

image_list = Image.objects.filter()

for some databases, like MySQL that are sometimes not good in optimizing subqueries that have are not dynamic, it might be better to first materialize the list of Item s, and then thus pass a list of ids in Django:

image_list = Image.objects.filter(item__in=item_list)

This is specified in the documentation as:

Be cautious about using nested queries and understand your database server's performance characteristics (if in doubt, benchmark!). Some database backends, most notably MySQL, don't optimize nested queries very well. It is more efficient, in those cases, to extract a list of values and then pass that into the second query.