testing/refactoring question (now with active_model_serializers)

question.rb

class ArticleController
  def comments
    comments = Comment.includes(:user).find_all_by_article_id(params[:id])
    render json: comments, root: false
  end
end

class CommentSerializer < ActiveModel::Serializer
  attributes :id, :created_at, :body, :subject

  class CommentUserSerializer < ActiveModel::Serializer
    attributes :id, :full_name, :avatar_url
  end

  has_one :user, serializer: CommentUserSerializer
end

# tests the url /articles/:id/comments

describe "GET comments" do
  it "renders the json of the comments for this article" do
    article = Fabricate(:article)
    comment1 = Fabricate(:comment, article: article)
    comment2 = Fabricate(:comment)
    get :comments, {:id => article.to_param}, valid_session

    # UGLYYYYYYY!!!!
    expected_json = "[" + CommentSerializer.new(comment1).to_json(:root => false) + "]"
    response.body.should == expected_json
  end
end

RSpec::Matchers.define :render_json_with do |key, value|
  match do |response|
    parsed_body = JSON.parse(response.body)
    parsed_body[key].should == value
  end
end