1 files changed, 76 insertions, 25 deletions

diff --git a/lib/gitlab/issuable_metadata.rb b/lib/gitlab/issuable_metadata.rb
index 6f760751b0f..e946fc00c4d 100644
--- a/lib/gitlab/issuable_metadata.rb
+++ b/lib/gitlab/issuable_metadata.rb
@@ -1,8 +1,52 @@
 # frozen_string_literal: true
 
 module Gitlab
-  module IssuableMetadata
-    def issuable_meta_data(issuable_collection, collection_type, user = nil)
+  class IssuableMetadata
+    include Gitlab::Utils::StrongMemoize
+
+    # data structure to store issuable meta data like
+    # upvotes, downvotes, notes and closing merge requests counts for issues and merge requests
+    # this avoiding n+1 queries when loading issuable collections on frontend
+    IssuableMeta = Struct.new(:upvotes, :downvotes, :user_notes_count, :mrs_count) do
+      def merge_requests_count(user = nil)
+        mrs_count
+      end
+    end
+
+    attr_reader :current_user, :issuable_collection
+
+    def initialize(current_user, issuable_collection)
+      @current_user = current_user
+      @issuable_collection = issuable_collection
+
+      validate_collection!
+    end
+
+    def data
+      return {} if issuable_ids.empty?
+
+      issuable_ids.each_with_object({}) do |id, issuable_meta|
+        issuable_meta[id] = metadata_for_issuable(id)
+      end
+    end
+
+    private
+
+    def metadata_for_issuable(id)
+      downvotes = group_issuable_votes_count.find { |votes| votes.awardable_id == id && votes.downvote? }
+      upvotes = group_issuable_votes_count.find { |votes| votes.awardable_id == id && votes.upvote? }
+      notes = grouped_issuable_notes_count.find { |notes| notes.noteable_id == id }
+      merge_requests = grouped_issuable_merge_requests_count.find { |mr| mr.first == id }
+
+      IssuableMeta.new(
+        upvotes.try(:count).to_i,
+        downvotes.try(:count).to_i,
+        notes.try(:count).to_i,
+        merge_requests.try(:last).to_i
+      )
+    end
+
+    def validate_collection!
       # ActiveRecord uses Object#extend for null relations.
       if !(issuable_collection.singleton_class < ActiveRecord::NullRelation) &&
           issuable_collection.respond_to?(:limit_value) &&
@@ -10,36 +54,43 @@ module Gitlab
 
         raise 'Collection must have a limit applied for preloading meta-data'
       end
+    end
 
-      # map has to be used here since using pluck or select will
-      # throw an error when ordering issuables by priority which inserts
-      # a new order into the collection.
-      # We cannot use reorder to not mess up the paginated collection.
-      issuable_ids = issuable_collection.map(&:id)
+    def issuable_ids
+      strong_memoize(:issuable_ids) do
+        # map has to be used here since using pluck or select will
+        # throw an error when ordering issuables by priority which inserts
+        # a new order into the collection.
+        # We cannot use reorder to not mess up the paginated collection.
+        issuable_collection.map(&:id)
+      end
+    end
 
-      return {} if issuable_ids.empty?
+    def collection_type
+      # Supports relations or paginated arrays
+      issuable_collection.try(:model)&.name ||
+        issuable_collection.first&.model_name.to_s
+    end
 
-      issuable_notes_count = ::Note.count_for_collection(issuable_ids, collection_type)
-      issuable_votes_count = ::AwardEmoji.votes_for_collection(issuable_ids, collection_type)
-      issuable_merge_requests_count =
+    def group_issuable_votes_count
+      strong_memoize(:group_issuable_votes_count) do
+        AwardEmoji.votes_for_collection(issuable_ids, collection_type)
+      end
+    end
+
+    def grouped_issuable_notes_count
+      strong_memoize(:grouped_issuable_notes_count) do
+        ::Note.count_for_collection(issuable_ids, collection_type)
+      end
+    end
+
+    def grouped_issuable_merge_requests_count
+      strong_memoize(:grouped_issuable_merge_requests_count) do
         if collection_type == 'Issue'
-          ::MergeRequestsClosingIssues.count_for_collection(issuable_ids, user)
+          ::MergeRequestsClosingIssues.count_for_collection(issuable_ids, current_user)
         else
           []
         end
-
-      issuable_ids.each_with_object({}) do |id, issuable_meta|
-        downvotes = issuable_votes_count.find { |votes| votes.awardable_id == id && votes.downvote? }
-        upvotes = issuable_votes_count.find { |votes| votes.awardable_id == id && votes.upvote? }
-        notes = issuable_notes_count.find { |notes| notes.noteable_id == id }
-        merge_requests = issuable_merge_requests_count.find { |mr| mr.first == id }
-
-        issuable_meta[id] = ::Issuable::IssuableMeta.new(
-          upvotes.try(:count).to_i,
-          downvotes.try(:count).to_i,
-          notes.try(:count).to_i,
-          merge_requests.try(:last).to_i
-        )
       end
     end
   end