SPEC: Add test cases for topic answer mixin

spec/serializers/topic_answer_mixin_spec.rb

@@ -0,0 +1,24 @@
+require 'rails_helper'
+
+describe TopicAnswerMixin do
+  let(:topic) { Fabricate(:topic) }
+  let(:post) { Fabricate(:post, topic: topic) }
+  let(:guardian) { Guardian.new }
+
+  before do
+    topic.custom_fields["accepted_answer_post_id"] = post.id
+    topic.save_custom_fields
+  end
+
+  it "should have true for `has_accepted_answer` field in each serializer" do
+    [
+      TopicListItemSerializer,
+      SearchTopicListItemSerializer,
+      SuggestedTopicSerializer,
+      UserSummarySerializer::TopicSerializer
+    ].each do |serializer|
+      json = serializer.new(topic, scope: guardian, root: false).as_json
+      expect(json[:has_accepted_answer]).to be_truthy
+    end
+  end  
+end