Find user who has no post in Rails

问题

This the the database relation:

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
end

I come across a functionality where I want to query all the users who don't have any posts yet. I know that we can do this with something like this:

users = User.all
users.each do |user|
  unless user.posts.any?
    # do something when user don't have any post.
  end
end

However, I wonder if there is any way to optimize this by using one query only.

Thanks!

回答1:

This results in a single query which fetches all users who don't have posts yet:

User.includes(:posts).references(:posts).where('posts.id IS NULL')

Another solution is this:

User.where('NOT EXISTS(SELECT 1 FROM posts WHERE user_id = users.id)')

Since this is a rather complex query to use everywhere, you can place this inside a named scope in User:

class User < ActiveRecord::Base
  scope :without_posts, -> { where('NOT EXISTS(SELECT 1 FROM posts WHERE user_id = users.id)') }
end

Now you can use this scope elsewhere in your application:

User.without_posts

回答2:

I'd try something like

User.joins(posts).where("count(posts.id) = 0")

Which returns all users that have 0 posts.

来源：https://stackoverflow.com/questions/25813171/find-user-who-has-no-post-in-rails