Question

I’m using Rails 3.2.8, and have models

class Subject < ActiveRecord::Base
  has_and_belongs_to_many :users
end
class User < ActiveRecord::Base
  has_and_belongs_to_many :subjects
end

in the SubjectsController, index

@subjects = Subject.includes(:users).all

I use includes to do eager load, and In the view file, I want to display all users of a subject by this:

<%= subject.users.count if not subject.users.nil? %>

the problem is I got db hit for every subject when display user count, which I think is the N+1 Query problem

anything I missed or did wrong?

btw: I’m using MySQL

Answer 1

The count method always generates SQL COUNT query. Since you have already loaded all the entries, try to use length to avoid extra query:

<%= subject.users.length if not subject.users.nil? %>