Mailboxer N+1 Query detected - ruby-on-rails

Question

I managed to install Mailboxer with this tutorial but i have a recurring error :
N+1 Query detected
Mailboxer::Message => [:sender]
Add to your finder: :includes => [:sender]
N+1 Query method call stack
app/views/conversations/_messages.html.erb:12:in `block in _app_views_conversations__messages_html_erb__3829465222244059655_69982701059040'
app/views/conversations/_messages.html.erb:1:in `_app_views_conversations__messages_html_erb__3829465222244059655_69982701059040'
app/views/conversations/show.html.erb:27:in `_app_views_conversations_show_html_erb__1439517360344897040_69982700516580'
app/views/conversations/_messages.html.erb:12:in `block in _app_views_conversations__messages_html_erb__3829465222244059655_69982701059040'
app/views/conversations/_messages.html.erb:1:in `_app_views_conversations__messages_html_erb__3829465222244059655_69982701059040'
app/views/conversations/show.html.erb:27:in `_app_views_conversations_show_html_erb__1439517360344897040_69982700516580'
I had another one for :message but i fixed the problem with includes. If i try to do the same with :sender, i have this error:
Association named 'sender' was not found on Mailboxer::Receipt; perhaps you misspelled it?
Tutorial original code : conversations_controller.rb
class ConversationsController < ApplicationController
before_action :authenticate_user!
def new
end
def create
recipients = User.where(id: conversation_params[:recipients])
conversation = current_user.send_message(recipients, conversation_params[:body], conversation_params[:subject]).conversation
flash[:success] = "Your message was successfully sent!"
redirect_to conversation_path(conversation)
end
def show
#receipts = conversation.receipts_for(current_user)
# mark conversation as read
conversation.mark_as_read(current_user)
end
def reply
current_user.reply_to_conversation(conversation, message_params[:body])
flash[:notice] = "Your reply message was successfully sent!"
redirect_to conversation_path(conversation)
end
def trash
conversation.move_to_trash(current_user)
redirect_to mailbox_inbox_path
end
def untrash
conversation.untrash(current_user)
redirect_to mailbox_inbox_path
end
private
def conversation_params
params.require(:conversation).permit(:subject, :body,recipients:[])
end
def message_params
params.require(:message).permit(:body, :subject)
end
end
Tutorial original code : show.html.erb
<div class="row">
<div class="spacer"></div>
<div class="col-md-6">
<%= link_to "Compose", new_conversation_path, class: "btn btn-success" %>
</div>
<div class="col-md-6 text-right">
<% if conversation.is_trashed?(current_user) %>
<%= link_to 'Untrash', untrash_conversation_path(conversation), class: 'btn btn-info', method: :post %>
<% else %>
<%= link_to 'Move to trash', trash_conversation_path(conversation), class: 'btn btn-danger', method: :post,
data: {confirm: 'Are you sure?'} %>
<% end %>
</div>
</div>
<div class="col-md-4">
<div class="panel panel-default">
<div class="panel-body">
<%= render 'mailbox/folders' %>
</div>
</div>
</div>
<div class="col-md-8">
<div class="panel panel-default">
<div class="panel-body">
<%= render partial: 'messages' %>
</div>
<div class="panel-footer">
<!-- Reply Form -->
<%= form_for :message, url: reply_conversation_path(conversation) do |f| %>
<div class="form-group">
<%= f.text_area :body, placeholder: "Reply Message", rows: 4, class: "form-control" %>
</div>
<%= f.submit "Reply", class: 'btn btn-danger pull-right' %>
<% end %>
<div class="clearfix"></div>
</div>
</div>
</div>
</div>
Tutorial original code : _messages.html.erb
<% #receipts.each do |receipt| %>
<% message = receipt.message %>
<div class="media">
<div class="media-left">
<!-- user avators can go here -->
<a href="#">
<img class="media-object" src="http://placehold.it/64x64" alt="...">
</a>
</div>
<div class="media-body">
<h4 class="media-heading">
<%= message.sender.username %> <br>
<small><b>Subject: </b><%= message.subject %></small><br>
<small><b>Date: </b><%= message.created_at.strftime("%A, %b %d, %Y at %I:%M%p") %></small>
</h4>
<%= message.body %>
</div>
</div>
<% end %>
If i delete <%= message.sender.username %> the problem is solved...
Any ideas ?

Answer 1

Normally, to fix these N+1 query problems we use an includes and by passing a hash, we can includes nested associations. So looking at this code, calling receipt.message.sender is triggering the error, so we have a Receipt model, a message association on it and a sender associated to that. So if we can find where we load the Receipt we can add includes(message: :user) like we would for any other model.
Digging into the mailboxer gem, the receipts_for method in your show action is just a wrapper for a couple of scopes on Mailboxer::Receipt. Since this method just runs some scopes for you, we can chain onto the end of it as if it were a normal ActiveRecord where chain.
So with all that in mind, we should be able to add on our includes and avoid the N+1 query problem, ending up with something like
#receipts = conversation.receipts_for(current_user).includes(message: :sender)