Rails has this great feature called counter_cache. When you have a has_many/belongs_to relationship between two models, it basically lets you say that you want to save the number of associated objects on the has_many side. So if you have Post has many Comments, you can do post#size and no SQL query is executed, Rails simply just looks at the comments_count field of the post.

Sometimes that field gets out of sync though, and if that happens, here is how to recalculate it!