Add rubocop Cop to detect activerecord .count and .each in haml file

Suggest to change `@AR_relation.count; @AR_relation.each...` to
`@AR_relation.load.size; @AR_relation.each...`. This will save one
database query

app/views/admin/deploy_keys/index.html.haml

 - page_title _('Deploy Keys')
 
 %h3.page-title.deploy-keys-title
-  = _('Public deploy keys (%{deploy_keys_count})') % { deploy_keys_count: @deploy_keys.count }
+  = _('Public deploy keys (%{deploy_keys_count})') % { deploy_keys_count: @deploy_keys.load.size }
   .float-right
     = link_to _('New deploy key'), new_admin_deploy_key_path, class: 'btn btn-success btn-sm btn-inverted'
 

app/views/profiles/emails/index.html.haml

@@ -18,7 +18,7 @@
         = f.submit _('Add email address'), class: 'btn btn-success', data: { qa_selector: 'add_email_address_button' }
     %hr
     %h4.prepend-top-0
-      = _('Linked emails (%{email_count})') % { email_count: @emails.count + 1 }
+      = _('Linked emails (%{email_count})') % { email_count: @emails.load.size + 1 }
     .account-well.append-bottom-default
       %ul
         %li

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

 - if @related_branches.any?
   %h2.related-branches-title
-    = pluralize(@related_branches.count, 'Related Branch')
+    = pluralize(@related_branches.size, 'Related Branch')
   %ul.unstyled-list.related-merge-requests
     - @related_branches.each do |branch|
       %li

app/views/projects/pages/_list.html.haml

 - verification_enabled = Gitlab::CurrentSettings.pages_domain_verification_enabled?
 
-- if can?(current_user, :update_pages, @project) && @domains.any?
+- if can?(current_user, :update_pages, @project) && @domains.load.any?
   .card
     .card-header
-      Domains (#{@domains.count})
+      Domains (#{@domains.size})
     %ul.list-group.list-group-flush.pages-domain-list{ class: ("has-verification-status" if verification_enabled) }
       - @domains.each do |domain|
         - domain = Gitlab::View::Presenter::Factory.new(domain, current_user: current_user).fabricate!

rubocop/cop/performance/ar_count_each.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      class ARCountEach < RuboCop::Cop::Cop
+        def message(ivar)
+          "If #{ivar} is AR relation, avoid `#{ivar}.count ...; #{ivar}.each... `, this will trigger two queries. " \
+          "Use `#{ivar}.load.size ...; #{ivar}.each... ` instead. If #{ivar} is an array, try to use #{ivar}.size."
+        end
+
+        def_node_matcher :count_match, <<~PATTERN
+          (send (ivar $_) :count)
+        PATTERN
+
+        def_node_matcher :each_match, <<~PATTERN
+          (send (ivar $_) :each)
+        PATTERN
+
+        def file_name(node)
+          node.location.expression.source_buffer.name
+        end
+
+        def in_haml_file?(node)
+          file_name(node).end_with?('.haml.rb')
+        end
+
+        def on_send(node)
+          return unless in_haml_file?(node)
+
+          ivar_count = count_match(node)
+          return unless ivar_count
+
+          node.each_ancestor(:begin) do |begin_node|
+            begin_node.each_descendant do |n|
+              ivar_each = each_match(n)
+
+              add_offense(node, location: :expression, message: message(ivar_count)) if ivar_each == ivar_count
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/performance/ar_count_each_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../support/helpers/expect_offense'
+require_relative '../../../../rubocop/cop/performance/ar_count_each.rb'
+
+describe RuboCop::Cop::Performance::ARCountEach do
+  include CopHelper
+  include ExpectOffense
+
+  subject(:cop) { described_class.new }
+
+  context 'when it is not haml file' do
+    it 'does not flag it as an offense' do
+      expect(subject).to receive(:in_haml_file?).with(anything).at_least(:once).and_return(false)
+
+      expect_no_offenses <<~SOURCE
+        show(@users.count)
+        @users.each { |user| display(user) }
+      SOURCE
+    end
+  end
+
+  context 'when it is haml file' do
+    before do
+      expect(subject).to receive(:in_haml_file?).with(anything).at_least(:once).and_return(true)
+    end
+
+    context 'when the same object uses count and each' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+        show(@users.count)
+             ^^^^^^^^^^^^ If @users is AR relation, avoid `@users.count ...; @users.each... `, this will trigger two queries. Use `@users.load.size ...; @users.each... ` instead. If @users is an array, try to use @users.size.
+        @users.each { |user| display(user) }
+        SOURCE
+
+        expect(cop.offenses.map(&:cop_name)).to contain_exactly('Performance/ARCountEach')
+      end
+    end
+
+    context 'when different object uses count and each' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses <<~SOURCE
+        show(@emails.count)
+        @users.each { |user| display(user) }
+        SOURCE
+      end
+    end
+
+    context 'when just using count without each' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses '@users.count'
+      end
+    end
+
+    context 'when just using each without count' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses '@users.each { |user| display(user) }'
+      end
+    end
+  end
+end

spec/views/projects/pages/show.html.haml_spec.rb

@@ -17,7 +17,7 @@ describe 'projects/pages/show' do
 
     assign(:project, project)
     allow(view).to receive(:current_user).and_return(user)
-    assign(:domains, [domain])
+    assign(:domains, project.pages_domains)
   end
 
   describe 'validation warning' do