Rails 3 finding parents which have no child

Question

In a one to many relationship with no counter cache how can I find parents with no child?

user.rb

has_many :pages

page.rb

belongs_to :user

I've tried

User.includes(:pages).where("pages.user_id is NULL")

This is making trouble in MySQL.

Answer 1

Try

User.joins("left join pages on pages.user_id = users.id").where("pages.user_id is null")