Optimize number of queries for approvals

This gets the number of queries down using some simple memoization and
eager loading queries.

changelogs/unreleased-ee/bvl-optimize-mr-approvals.yml

+---
+title: Speed up approvals calculations
+merge_request: 4492
+author:
+type: performance

ee/app/controllers/ee/projects/merge_requests_controller.rb

@@ -3,6 +3,8 @@ module EE
     module MergeRequestsController
       extend ActiveSupport::Concern
 
+      APPROVAL_RENDERING_ACTIONS = [:approve, :approvals, :unapprove].freeze
+
       def approve
         unless merge_request.can_approve?(current_user)
           return render_404
@@ -31,6 +33,21 @@ module EE
 
       protected
 
+      # rubocop:disable Gitlab/ModuleWithInstanceVariables
+      # Assigning both @merge_request and @issuable like in
+      # `Projects::MergeRequests::ApplicationController`, and calling super if
+      # we don't need the extra includes requires us to disable this cop.
+      def merge_request
+        return super unless APPROVAL_RENDERING_ACTIONS.include?(action_name.to_sym)
+
+        @issuable = @merge_request ||= project.merge_requests
+                                         .includes(approvals: :user,
+                                                   approvers: :user,
+                                                   approver_groups: :group)
+                                         .find_by!(iid: params[:id])
+      end
+      # rubocop:disable Gitlab/ModuleWithInstanceVariables
+
       def define_edit_vars
         super
 

ee/app/models/concerns/approvable.rb

@@ -13,13 +13,13 @@ module Approvable
   #
   def approvals_left
     [
-      [approvals_required - approvals.count, number_of_potential_approvers].min,
+      [approvals_required - approvals.size, number_of_potential_approvers].min,
       0
     ].max
   end
 
   def approvals_required
-    approvals_before_merge || target_project.approvals_before_merge
+    @approvals_required ||= approvals_before_merge || target_project.approvals_before_merge
   end
 
   def approvals_before_merge