Question

I have three models:

class Book < ActiveRecord::Base
  has_many :collections
  has_many :users, :through => :collections
end

class User < ActiveRecord::Base
  has_many :collections
  has_many :books, :through => :collections
end

class Collection < ActiveRecord::Base
  belongs_to :book
  belongs_to :user
end

I’m trying to display a list of the books and have a link to either add or remove from the user’s collection. I can’t quite figure out the best syntax to do this.

For example, if I do the following:

Controller

class BooksController < ApplicationController
  def index
    @books = Book.all
  end
end

View

...
<% if book.users.include?(current_user) %>
...

or obviously the inverse…

...
<% if current_user.books.include?(book) %>
...

Then queries are sent for each book to check on that include? which is wasteful. I was thinking of adding the users or collections to the :include on the Book.all, but I’m not sure this is the best way. Effectively all I need is the book object and just a boolean column of whether or not the current user has the book in their collection, but I’m not sure how to forumlate the query in order to do that.

Thanks in advance for your help.

-Damien

Answer 1

I have created a gem(select_extra_columns) for returning join/calculated/aggregate columns in a ActiveRecord finders. Using this gem, you will be able to get the book details and the flag indicating if the current user has the book in one query.

In your User model register the select_extra_columns feature.

class Book < ActiveRecord::Base
  select_extra_columns
  has_many :collections
  has_many :users, :through => :collections
end

Now in your controller add this line:

@books = Book.all(
           :select => "books.*, IF(collections.id, 1, 0) AS belongs_to_user",
           :extra_columns => {:belongs_to_user => :boolean},
           :joins => "LEFT OUTER JOIN collections 
                        ON book.id = collections.book_id AND 
                           collections.user_id = #{current_user.id}"
          )

Now in your view you can do the following.

book.belongs_to_user?