Unit test - average_rating

product.rb

class Product < ActiveRecord::Base
  has_many :orders
  has_many :comments
  
  def average_rating
    comments.average(:rating).to_f
  end
  validates :name, presence: true
end

product_spec.rb

require 'rails_helper' # also requires spec_helper and adds other stuff - if didnt need Rails just use spec_helper but unlikely

describe Product do
  
  describe "#average_rating" do # - the '#' signifies that we are testing an instance method - describe the method you will be testing (which belongs to the Class)
    
    #context 1, test 1
    context "when the product doesn't have any comments" do #create a context to test
      before do # before running the test... 
        @product = Product.new # ...set up a new instance - this one has no attributes, therefore no comments are present
      end
      
      it 'returns 0.0' do # it refers to the instance just created. Describe what you are testing for.
        expect(@product.average_rating).to eq 0.0
      end
    end

    #context 2, test 2
    context "when the product has comments" do # create context
      before do # before running the test...
        @product = Product.create 
        @product.comments.new(rating: 1)
        @product.comments.new(rating: 3)
        @product.comments.new(rating: 5)
      end
      
      it 'returns the average rating of all comments' do
        expect(@product.average_rating).to eq 3
      end
    end
    
    #context 3, test 3
    context "when one of the product's comments does not have a rating" do
      before do
        @product = Product.new
        @product.comments.new(rating: '')
      end
        
      it 'returns 0.0' do
        expect(@product.average_rating).to eq 0.0
      end
    end
    
  end #end describe #average_rating     
  
end #end describe Product