Include associated model for all objects (in index action)

I am trying to develop ratings for my application, where a User is able to set a specific rating for a comment. I have followed the following tutorial in order to do so.

Here are my associations:

class Rating < ActiveRecord::Base
  belongs_to :comment
  belongs_to :user
end

class Comment < ActiveRecord::Base
  has_many :ratings
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :ratings
  has_many :comments
end

My problem here is that, in the index action of my comments controller, I need to include the rating that the user has done for that comment. In the tutorial is just shown how to select a particular rating by doing this:

@rating = Rating.where(comment_id: @comment.id, user_id: @current_user.id).first 

unless @rating 
  @rating = Rating.create(comment_id: @comment.id, user_id: @current_user.id, score: 0) 
end

However, I will have several ratings, because in my controller I have:

def index
  @comments = @page.comments #Here each comment should have the associated rating for the current_user, or a newly created rating if it does not exist.
end

Solution

You want to find the comment's rating where the rating's user_id matches the current user.

<%= comment.ratings.where(user_id: current_user.id).first %>

However this sort of logic is pretty cumbersome in the views, a better strategy would be to define a scope in Rating that returns all ratings made by a specific user.

class Rating
  scope :by_user, lambda { |user| where(user_id: user.id) }
end

class Comment
  # this will return either the rating created by the given user, or nil
  def rating_by_user(user)
    ratings.by_user(user).first 
  end
end

Now in your view, you have access to the rating for the comment created by the current user:

<% @comments.each do |comment| %>
  <%= comment.rating_by_user(current_user) %>
<% end %>

If you want to eager load all ratings in your index page, you can do the following:

def index
  @comments = page.comments.includes(:ratings)
end

You can then find the correct rating with the following:

<% @comments.each do |comment| %>
  <%= comment.ratings.find { |r| r.user_id == current_user.id } %>
<% end %>

This would return the correct rating without generating any extra SQL queries, at the expense of loading every associated rating for each comment.

I'm not aware of a way in ActiveRecord to eager load a subset of a has_many relationship. See this related StackOverflow question, as well as this blog post that contains more information about eager loading.