How can I get the number of records that reference a particular foreign key in Django?

Question

I'm working on a blog application in Django. Naturally, I have models set up such that there are Posts and Comments, and a particular Post may have many Comments; thus, Post is a ForeignKey in the Comments model.

Given a Post object, is there an easy way (ideally, through a method call) to find out how many Comments belong to the Post?

Answer 1

Comments.objects.filter(post=post).count()

or:

post.comment_set.count()

Answer 2

You can add field CommentCount to you Post model, and update it in pre_save, pre_delete signals. It's a hard for the db to calculate comments count at every view call and number of queries will be grow.