issuable#labels_array explicitly load the labels

This will be useful when you want to ask for the 
number of items and later iterate over them, without 
needing to ask if the association is load or not.

So you avoid extra database queries

CHANGELOG

@@ -30,6 +30,7 @@ v 8.9.0 (unreleased)
   - Add Application Setting to configure Container Registry token expire delay (default 5min)
   - Cache assigned issue and merge request counts in sidebar nav
   - Cache project build count in sidebar nav
+  - Reduce number of queries needed to render issue labels in the sidebar
 v 8.8.3
   - Fix incorrect links on pipeline page when merge request created from fork

app/models/concerns/issuable.rb

@@ -213,6 +213,10 @@ module Issuable
     hook_data
   end
+  def labels_array
+    labels.to_a
+  end
+
   def label_names
     labels.order('title ASC').pluck(:title)
   end

app/views/shared/issuable/_sidebar.html.haml

@@ -114,20 +114,20 @@
           .sidebar-collapsed-icon
             = icon('tags')
             %span
-              = issuable.labels.count
+              = issuable.labels_array.size
           .title.hide-collapsed
             Labels
             = icon('spinner spin', class: 'block-loading')
             - if can_edit_issuable
               = link_to 'Edit', '#', class: 'edit-link pull-right'
-          .value.bold.issuable-show-labels.hide-collapsed{ class: ("has-labels" if issuable.labels.any?) }
-            - if issuable.labels.any?
-              - issuable.labels.each do |label|
+          .value.bold.issuable-show-labels.hide-collapsed{ class: ("has-labels" if issuable.labels_array.any?) }
+            - if issuable.labels_array.any?
+              - issuable.labels_array.each do |label|
                 = link_to_label(label, type: issuable.to_ability_name)
             - else
               .light None
           .selectbox.hide-collapsed
-            - issuable.labels.each do |label|
+            - issuable.labels_array.each do |label|
               = hidden_field_tag "#{issuable.to_ability_name}[label_names][]", label.id, id: nil
             .dropdown
               %button.dropdown-menu-toggle.js-label-select.js-multiselect{type: "button", data: {toggle: "dropdown", field_name: "#{issuable.to_ability_name}[label_names][]", ability_name: issuable.to_ability_name, show_no: "true", show_any: "true", project_id: (@project.id if @project), issue_update: issuable_json_path(issuable), labels: (namespace_project_labels_path(@project.namespace, @project, :json) if @project)}}

spec/models/concerns/issuable_spec.rb

@@ -227,6 +227,20 @@ describe Issue, "Issuable" do
     end
   end
+  describe '#labels_array' do
+    let(:project) { create(:project) }
+    let(:bug) { create(:label, project: project, title: 'bug') }
+    let(:issue) { create(:issue, project: project) }
+
+    before(:each) do
+      issue.labels << bug
+    end
+
+    it 'loads the association and returns it as an array' do
+      expect(issue.reload.labels_array).to eq([bug])
+    end
+  end
+
   describe "votes" do
     let(:project) { issue.project }