Rails select distinct association from collection

Question

Let's say I have a model, Post, and a model, User. Given a collection of posts (@posts), how can I get the unique authors?

This works, but is terrible for obvious reasons: @posts.map(&:author).uniq(&:id).

A slightly better method is: User.where(id: @posts.pluck(:author_id)).

Is there a better way?

Answer 1

User.where(id: @posts.select('distinct author_id'))

OR

User.where(id: @posts.pluck('distinct author_id'))

Will apply 'DISTINCT' in SQL query (not as Array#uniq)

Note:

If you are trying to get users activerecord relation, 'Distinct' is useless because 'ID' is already unique in users table and you won't get any duplicated user.

But if you need only array of unique author_ids you can use @posts.map(&:author_id).uniq