Cache Participable#participants in instance variable

CHANGELOG

@@ -135,6 +135,7 @@ v 8.9.0 (unreleased)
   - Update tanuki logo highlight/loading colors
   - Remove explicit Gitlab::Metrics.action assignments, are already automatic.
   - Use Git cached counters for branches and tags on project page
+  - Cache participable participants in an instance variable.
   - Filter parameters for request_uri value on instrumented transactions.
   - Remove duplicated keys add UNIQUE index to keys fingerprint column
   - ExtractsPath get ref_names from repository cache, if not there access git.

app/models/concerns/participable.rb

@@ -53,6 +53,16 @@ module Participable
   #
   # Returns an Array of User instances.
   def participants(current_user = nil)
+    @participants ||= Hash.new do |hash, user|
+      hash[user] = raw_participants(user)
+    end
+
+    @participants[current_user]
+  end
+
+  private
+
+  def raw_participants(current_user = nil)
     current_user ||= author
     ext = Gitlab::ReferenceExtractor.new(project, current_user)
     participants = Set.new

spec/models/concerns/participable_spec.rb

@@ -37,6 +37,16 @@ describe Participable, models: true do
       expect(participants).to include(user3)
     end
 
+    it 'caches the raw list of participants' do
+      instance = model.new
+      user1 = build(:user)
+
+      expect(instance).to receive(:raw_participants).once
+
+      instance.participants(user1)
+      instance.participants(user1)
+    end
+
     it 'supports attributes returning another Participable' do
       other_model = Class.new { include Participable }