1 files changed, 45 insertions, 0 deletions
diff --git a/rubocop/cop/performance/ar_count_each.rb b/rubocop/cop/performance/ar_count_each.rb
new file mode 100644
index 00000000000..2fe8e549872
--- /dev/null
+++ b/rubocop/cop/performance/ar_count_each.rb
@@ -0,0 +1,45 @@
+# 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