Improve performance for issue#show page

* store @participants in variable
* store result of subscribed? call into variable

In total it reduce amount of SQL queries for issue with 10 comments/participants twice.
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/issues_controller.rb

@@ -55,6 +55,7 @@ class Projects::IssuesController < Projects::ApplicationController
   end
 
   def show
+    @participants = @issue.participants(current_user, @project)
     @note = @project.notes.new(noteable: @issue)
     @notes = @issue.notes.inc_author.fresh
     @noteable = @issue

app/models/concerns/participable.rb

@@ -35,6 +35,8 @@ module Participable
     end
   end
 
+  # Be aware that this method makes a lot of sql queries.
+  # Save result into variable if you are going to reuse it inside same request
   def participants(current_user = self.author, project = self.project)
     participants = self.class.participant_attrs.flat_map do |attr|
       meth = method(attr)

app/views/projects/issues/_discussion.html.haml

@@ -12,8 +12,8 @@
     .votes-holder.pull-right
       #votes= render 'votes/votes_block', votable: @issue
     .participants
-      %span= pluralize(@issue.participants(current_user).count, 'participant')
-      - @issue.participants(current_user).each do |participant|
+      %span= pluralize(@participants.count, 'participant')
+      - @participants.each do |participant|
         = link_to_member(@project, participant, name: false, size: 24)
     .voting_notes#notes= render 'projects/notes/notes_with_form'
   %aside.col-md-3

app/views/projects/issues/_issue_context.html.haml

@@ -28,18 +28,19 @@
       = f.submit class: 'btn'
 
   - if current_user
+    - subscribed = @issue.subscribed?(current_user)
     %div.prepend-top-20.clearfix
       .issuable-context-title
         %label
           Subscription:
       %button.btn.btn-block.subscribe-button{:type => 'button'}
         %i.fa.fa-eye
-        %span= @issue.subscribed?(current_user) ? "Unsubscribe" : "Subscribe"
-      - subscribtion_status = @issue.subscribed?(current_user) ? "subscribed" : "unsubscribed"
+        %span= subscribed ? "Unsubscribe" : "Subscribe"
+      - subscribtion_status = subscribed ? "subscribed" : "unsubscribed"
       .subscription-status{"data-status" => subscribtion_status}
-        .description-block.unsubscribed{class: ( "hidden" if @issue.subscribed?(current_user) )}
+        .description-block.unsubscribed{class: ( "hidden" if subscribed )}
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-        .description-block.subscribed{class: ( "hidden" unless @issue.subscribed?(current_user) )}
+        .description-block.subscribed{class: ( "hidden" unless subscribed )}
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