Rails: Order array by number of child objects

Question

I have an array of posts. I want to order this array of posts by number of comments they have (First object in the array being most comments, last being the least). A Post has many comments. This seems like a simple problem, but I can't figure out how to order them. I've tried and I don't believe it's possible to achieve this via the order method. Is there a Rails method I don't know about? Or will I have to home roll it? Thanks in advance!

PS: My relation of comments to posts is polymorphic Comment - (belongs_to :threadable)

Answer 1

You can do this with a complicated active record query, however, the easiest thing by far is to just add a :counter_cache on association. You just add a field on your Post model called comments_count and then in your Comment model:

belongs_to :post, counter_cache: true

Then since it's just a column like all your other columns you can order by it.

See: http://guides.rubyonrails.org/association_basics.html

Answer 2

The easiest way to do this would probably be using the counter_cache option

see:

• http://guides.rubyonrails.org/association_basics.html#counter-cache
• http://railscasts.com/episodes/23-counter-cache-column

-

class Comment
  belongs_to :post, counter_cache: true
end

add the column comments_count to the posts database table, then order by that

Answer 3

The best way to achieve it is to use counter cache feature. You need to add integer column to your posts table called comments_count. Then on your comments model you need to change your post association to read:

 belongs_to :post, counter_cache: true

Rails will handle comments_count column to show how many comments are associated with given post. You can then use this column for ordering.