Is there a more efficient way than this to load records associated with records in a list? - sql

Question

I have a model Playlist, and a model User, both of which have_many of each other, through a join model PlaylistUser.
On my playlists#show action, I want to print a list of all of a Playlist's Users, along with the first two Playlists associated with each of those Users.
Right now here's what I have:
playlists/show.html.erb
<% #playlist = Playlist.find(params[:id]) %>
<% #playlist.users.each do |user| %>
<%= user.name %>
<%= user.playlists.first.name %>
<%= user.playlists.second.name %>
<% end %>
models
class User < ActiveRecord::Base
has_many :playlist_users
has_many :playlists, :through => :playlist_users
end
class PlaylistUser < ActiveRecord::Base
belongs_to :playlist
belongs_to :user
end
class Playlist < ActiveRecord::Base
has_many :playlist_users
has_many :users, :through => :playlist_users
end
But there is an enormous change in performance when I delete the user.playlists lines and print out only the user.name, because then the database only has to make one query, as opposed to hundreds.
Does anyone know of a way to make this more efficient? Maybe I could somehow load all the associated Playlists in the original query?

Answer 1

You can use the includes method to tell Rails to preload associated records with just one query upfront.
Loading from the database is a controller responsibility and should not happen in the view. Therefore, add the following to your controller:
playlist = Playlist.find(params[:id])
#users = playlist.users.includes(:playlists)
And change your view to just iterate over the users array:
<% #users.each do |user| %>
<%= user.name %>
<%= user.playlists.first.name %>
<%= user.playlists.second.name %>
<% end %>