Q:

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

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 2014-06-30 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)

A:

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

