What I stumbled upon in the ActiveModel :: Serializer test

Introduction

ActiveModel :: Serializer Rspec gave an example of how to write a test. However, I've stumbled upon writing a test for another Serializer, so I'll post it.

Source code

In this example, assume that Article and Category have a many-to-many relationship and have an intermediate model called ʻarticle_category.rb`.

article.rb

class Article < ApplicationRecord
  has_many :article_categories
  has_many :categories, through: :article_categories
end

article_serializer.rb

class ArticleSerializer < ActiveModel::Serializer
  attributes :id, :title
  has_many :categories
end

When I write the test of this Serializer, it looks like this

article_serializer_spec.rb

require 'rails_helper'

RSpec.describe ArticleSerializer, type: :serializer do
  context "When a new article is created" do
    let(:article) { create(:article) }
    let(:category) { create(:category) }
    let(:article_category) do
      create(:article_category, category: category, article: article)
    end

    it "Serialized JSON is created" do
      serializer = ArticleSerializer.new(article)
      expect(serializer.to_json).to eq(article.to_json(:only => [:id, :title], :include => { :category })
    end
  end
end

test results

When I tested it, the returned JSON didn't contain the has_many related category and was left empty. why,,,

Solution

It seems that the way this test was written was bad, so I wrote it like this and it passed.

article_serializer_spec.rb

require 'rails_helper'

RSpec.describe ArticleSerializer, type: :serializer do
  context "When a new article is created" do
    let!(:article) { create(:article) }
    let!(:category) { create(:category) }
    let!(:article_category) do
      create(:article_category, category: category, article: article)
    end

    it "Serialized JSON is created" do
      serializer = ArticleSerializer.new(article)
      expect(serializer.to_json).to eq(article.to_json(:only => [:id, :title], :include => { :category })
    end
  end
end

The change is that let is changed tolet!. If you leave let as it is, the only object created in the example is ʻarticle. The reason is that let` creates an object only when it appears in the example.

So it's a good idea to use let! In an example like this!