Optimize queries for snippet listings

- Avoid N+1 queries for authors and comment counts
- Avoid an additional snippet existence query

2 files changed, 35 insertions, 2 deletions

diff --git a/lib/gitlab/issuable_metadata.rb b/lib/gitlab/issuable_metadata.rb
index be73bcd5506..6f760751b0f 100644
--- a/lib/gitlab/issuable_metadata.rb
+++ b/lib/gitlab/issuable_metadata.rb
@@ -19,7 +19,7 @@ module Gitlab
 
       return {} if issuable_ids.empty?
 
-      issuable_note_count = ::Note.count_for_collection(issuable_ids, collection_type)
+      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 =
         if collection_type == 'Issue'
@@ -31,7 +31,7 @@ module Gitlab
       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_note_count.find { |notes| notes.noteable_id == id }
+        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(
diff --git a/lib/gitlab/noteable_metadata.rb b/lib/gitlab/noteable_metadata.rb
new file mode 100644
index 00000000000..f3f8933b81f
--- /dev/null
+++ b/lib/gitlab/noteable_metadata.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module NoteableMetadata
+    def noteable_meta_data(noteable_collection, collection_type)
+      # ActiveRecord uses Object#extend for null relations.
+      if !(noteable_collection.singleton_class < ActiveRecord::NullRelation) &&
+          noteable_collection.respond_to?(:limit_value) &&
+          noteable_collection.limit_value.nil?
+
+        raise 'Collection must have a limit applied for preloading meta-data'
+      end
+
+      # map has to be used here since using pluck or select will
+      # throw an error when ordering noteables which inserts
+      # a new order into the collection.
+      # We cannot use reorder to not mess up the paginated collection.
+      noteable_ids = noteable_collection.map(&:id)
+
+      return {} if noteable_ids.empty?
+
+      noteable_notes_count = ::Note.count_for_collection(noteable_ids, collection_type)
+
+      noteable_ids.each_with_object({}) do |id, noteable_meta|
+        notes = noteable_notes_count.find { |notes| notes.noteable_id == id }
+
+        noteable_meta[id] = ::Noteable::NoteableMeta.new(
+          notes.try(:count).to_i
+        )
+      end
+    end
+  end
+end