Use cached values to compute total issues count in milestone index pages

Milestoneish#issues_visible_to_user caches only the ActiveRecord relation,
not the actual result.
Introduce Milestoneish#total_issues_count that relies on the cached method
Milestoneish#count_issues_by_state to reduce the number of queries.

app/models/concerns/milestoneish.rb

@@ -7,11 +7,14 @@ module Milestoneish
 
   def total_items_count(user)
     memoize_per_user(user, :total_items_count) do
-      issues_count = count_issues_by_state(user).values.sum
-      issues_count + merge_requests.size
+      total_issues_count(user) + merge_requests.size
     end
   end
 
+  def total_issues_count(user)
+    count_issues_by_state(user).values.sum
+  end
+
   def complete?(user)
     total_items_count(user) > 0 && total_items_count(user) == closed_items_count(user)
   end

app/views/shared/milestones/_milestone.html.haml

@@ -9,7 +9,7 @@
       .pull-right.light #{milestone.percent_complete(current_user)}% complete
   .row
     .col-sm-6
-      = link_to pluralize(milestone.issues_visible_to_user(current_user).size, 'Issue'), issues_path
+      = link_to pluralize(milestone.total_issues_count(current_user), 'Issue'), issues_path
       ·
       = link_to pluralize(milestone.merge_requests.size, 'Merge Request'), merge_requests_path
     .col-sm-6= milestone_progress_bar(milestone)

changelogs/unreleased/reduce-queries-milestone-index.yml

+---
+title: Use cached values to compute total issues count in milestone index pages
+merge_request: 8518
+author: