Better way of checking if the current user already like a post

Question

I have three models which look like this:

class Post < ActiveRecord::Base
 belongs_to :user, :counter_cache => :count_post
 has_many :likes
end
class User < ActiveRecord::Base
 has_many :posts
 has_many :likes
end
class Like < ActiveRecord::Base
 belongs_to :user
 belongs_to :post, :counter_cache => :count_like
end

A user can only like once per Post so I have a method that checks if the current user already like a post and the dislike link is displayed.

<% if post.has_like_from? current_user %>
 <li id="like_<%= dom_id(post) %>"><%= link_to "like", create_like_path(:post_id => post.id), :remote => true %></li>
<% else %>
 <li id="dislike_<%= dom_id(post) %>"><%= render 'dislike', :post => post %></li>
<% end %>

And I have this method on model post:

 def has_like_from?(target_user)
 likes.where(:user_id => target_user.id).first == nil
 end

Is there any way to check every post if current user already like a post? I want to increase performance because load to many posts per request.

Example: startrace when load 7 records

Started GET "/" for 127.0.0.1 at 2014年06月30日 23:32:51 +0700
 ←[1m←[35mUser Load (1.0ms)←[0m SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
Processing by HomesController#index as HTML
 ←[1m←[36mPost Load (1.0ms)←[0m ←[1mSELECT "posts".* FROM "posts" ORDER BY created_at DESC←[0m
 ←[1m←[35mLike Load (1.0ms)←[0m SELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 7 AND "lik
es"."user_id" = 1 LIMIT 1
 ←[1m←[36mLike Load (0.0ms)←[0m ←[1mSELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 6 AND
"likes"."user_id" = 1 LIMIT 1←[0m
 ←[1m←[35mLike Load (1.0ms)←[0m SELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 5 AND "lik
es"."user_id" = 1 LIMIT 1
 ←[1m←[36mLike Load (1.0ms)←[0m ←[1mSELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 4 AND
"likes"."user_id" = 1 LIMIT 1←[0m
 ←[1m←[35mLike Load (0.0ms)←[0m SELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 3 AND "lik
es"."user_id" = 1 LIMIT 1
 ←[1m←[36mLike Load (0.0ms)←[0m ←[1mSELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 2 AND
"likes"."user_id" = 1 LIMIT 1←[0m
 ←[1m←[35mLike Load (0.0ms)←[0m SELECT "likes".* FROM "likes" WHERE "likes"."post_id" = 1 AND "lik
es"."user_id" = 1 LIMIT 1
 Rendered homes/_posts.html.erb (29.0ms)

Answer 1

Have you created an index for likes.post_id? Note also that you can simplify that method by using has_many with the option through and ActiveRecord#include?. That's what I'd write (with positive logic, I don't see why has_like_from? should perform a == nil):

class Post < ActiveRecord::Base
 belongs_to :user, :counter_cache => :count_post
 has_many :likes
 has_many :like_users, :through => :likes
 def liked_by?(user)
 like_users.include?(user)
 end
end

• rui: i think this will load whole like_users table to memory, it may be more efficient to do like_users.exists?(user_id: user.id).
• tokland: @rui, Here #include? comes from active-record, not Array, it should generate the same SQL than your code. I cannot test right now, but I'd be surprised if that's not the case.