Add rubocop for AR exists? and present?/blank?

app/views/projects/labels/index.html.haml

@@ -22,13 +22,13 @@
       .content-list.manage-labels-list.js-prioritized-labels{ data: { url: set_priorities_project_labels_path(@project), sortable: can_admin_label } }
         #js-priority-labels-empty-state.priority-labels-empty-state{ class: "#{'hidden' unless @prioritized_labels.empty? && search.blank?}" }
           = render 'shared/empty_states/priority_labels'
-        - if @prioritized_labels.present?
+        - if @prioritized_labels.any?
           = render partial: 'shared/label', collection: @prioritized_labels, as: :label, locals: { force_priority: true, subject: @project }
         - elsif search.present?
           .nothing-here-block
             = _('No prioritized labels with such name or description')
 
-    - if @labels.present?
+    - if @labels.any?
       .other-labels
         %h5{ class: ('hide' if hide) }= _('Other Labels')
         .content-list.manage-labels-list.js-other-labels

rubocop/cop/performance/ar_exists_and_present_blank.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Performance
+      class ARExistsAndPresentBlank < RuboCop::Cop::Cop
+        def message_present(ivar)
+          "Avoid `#{ivar}.present?`, because it will generate database query 'Select TABLE.*' which is expensive. "\
+          "Suggest to use `#{ivar}.any?` to replace `#{ivar}.present?`"
+        end
+
+        def message_blank(ivar)
+          "Avoid `#{ivar}.blank?`, because it will generate database query 'Select TABLE.*' which is expensive. "\
+          "Suggest to use `#{ivar}.empty?` to replace `#{ivar}.blank?`"
+        end
+
+        def_node_matcher :exists_match, <<~PATTERN
+          (send (ivar $_) :exists?)
+        PATTERN
+
+        def_node_matcher :present_match, <<~PATTERN
+          (send (ivar $_) :present?)
+        PATTERN
+
+        def_node_matcher :blank_match, <<~PATTERN
+          (send (ivar $_) :blank?)
+        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_present = present_match(node)
+          ivar_blank = blank_match(node)
+          return unless ivar_present || ivar_blank
+
+          node.each_ancestor(:begin) do |begin_node|
+            begin_node.each_descendant do |n|
+              ivar_exists = exists_match(n)
+              next unless ivar_exists
+
+              add_offense(node, location: :expression, message: message_present(ivar_exists)) if ivar_exists == ivar_present
+              add_offense(node, location: :expression, message: message_blank(ivar_exists)) if ivar_exists == ivar_blank
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/performance/ar_exists_and_present_blank_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require_relative '../../../support/helpers/expect_offense'
+require_relative '../../../../rubocop/cop/performance/ar_exists_and_present_blank.rb'
+
+describe RuboCop::Cop::Performance::ARExistsAndPresentBlank 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
+        return unless @users.exists?
+        show @users if @users.present?
+      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 'the same object uses exists? and present?' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+        return unless @users.exists?
+        show @users if @users.present?
+                       ^^^^^^^^^^^^^^^ Avoid `@users.present?`, because it will generate database query 'Select TABLE.*' which is expensive. Suggest to use `@users.any?` to replace `@users.present?`
+        SOURCE
+
+        expect(cop.offenses.map(&:cop_name)).to contain_exactly('Performance/ARExistsAndPresentBlank')
+      end
+    end
+
+    context 'the same object uses exists? and blank?' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+        return unless @users.exists?
+        show @users if @users.blank?
+                       ^^^^^^^^^^^^^ Avoid `@users.blank?`, because it will generate database query 'Select TABLE.*' which is expensive. Suggest to use `@users.empty?` to replace `@users.blank?`
+        SOURCE
+
+        expect(cop.offenses.map(&:cop_name)).to contain_exactly('Performance/ARExistsAndPresentBlank')
+      end
+    end
+
+    context 'the same object uses exists?, blank? and present?' do
+      it 'flags it as an offense' do
+        expect_offense <<~SOURCE
+        return unless @users.exists?
+        show @users if @users.blank?
+                       ^^^^^^^^^^^^^ Avoid `@users.blank?`, because it will generate database query 'Select TABLE.*' which is expensive. Suggest to use `@users.empty?` to replace `@users.blank?`
+        show @users if @users.present?
+                       ^^^^^^^^^^^^^^^ Avoid `@users.present?`, because it will generate database query 'Select TABLE.*' which is expensive. Suggest to use `@users.any?` to replace `@users.present?`
+        SOURCE
+
+        expect(cop.offenses.map(&:cop_name)).to contain_exactly('Performance/ARExistsAndPresentBlank', 'Performance/ARExistsAndPresentBlank')
+      end
+    end
+
+    RSpec.shared_examples 'different object uses exists? and present?/blank?' do |another_method|
+      it 'does not flag it as an offense' do
+        expect_no_offenses <<~SOURCE
+        return unless @users.exists?
+        present @emails if @emails.#{another_method}
+        SOURCE
+      end
+    end
+
+    it_behaves_like 'different object uses exists? and present?/blank?', 'present?'
+    it_behaves_like 'different object uses exists? and present?/blank?', 'blank?'
+
+    RSpec.shared_examples 'Only using one present?/blank? without exists?' do |non_exists_method|
+      it 'does not flag it as an offense' do
+        expect_no_offenses "@users.#{non_exists_method}"
+      end
+    end
+
+    it_behaves_like 'Only using one present?/blank? without exists?', 'present?'
+    it_behaves_like 'Only using one present?/blank? without exists?', 'blank?'
+
+    context 'when using many present?/empty? without exists?' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses <<~SOURCE
+        @user.present?
+        @user.blank?
+        @user.present?
+        @user.blank?
+        SOURCE
+      end
+    end
+
+    context 'when just using exists? without present?/blank?' do
+      it 'does not flag it as an offense' do
+        expect_no_offenses '@users.exists?'
+
+        expect_no_offenses <<~SOURCE
+        @users.exists?
+        @users.some_other_method?
+        @users.exists?
+        SOURCE
+      end
+    end
+  end
+end