Assigns in Ruby on Rails — How you can test instance variables in views and controllers

has special helper methods that allow you to test instance variables in your app’s and . These are assign and assigns, which stores a hash of the instance variables you can access in your tests. Ruby on Rails views controllers RSpec This tutorial will guide you through how to write a simple controller and view with corresponding tests. These will cover assigning instance variables depending on whether certain conditions are met. How assign and assigns work Controller tests View tests Learn more How assign and assigns works If you want to set a controller or view’s instance variables in your RSpec test, then call assign either in a before block or at the start of an example group. The first argument is the name of the instance variable while the second is the value you want to assign to it. before do assign(:message, 'great test!') end Call its counterpart assigns to examine the instance variables that have been set. assigns(:message) => 'great test!' Note: According to RSpec-Rails , in Rails 5.x, controller testing has been moved to its own gem which is rails-controller-testing . Using assigns in your controller specs without adding this gem will no longer work. Controller tests Ruby on Rails RSpec includes the helper method controller to call the being tested. Controller Here is an example of a controller action that sets an instance variable depending on parameters sent to it. class MessageController < ApplicationController def show if params[:message] @message = params[:message] end end end Then you can test this using RSpec. Divide your tests into contexts for the conditions that determine whether the assignment should happen. The first one is when the message is missing from the parameters. context 'when params[:message] is not present' do before { post :show } it 'does not assign a message' do expect(assigns(:message)).to be_nil end end And this is the test context for when the message is present in the parameters posted to the controller action. context 'when params[:message] is present' do before { post :show, { message: 'great test!' } } it 'assigns the message from params' do expect(assigns(:message)).to eq ‘great test!’ end end View tests To test , you place the test in the matching spec path as the the view in the app folder, then start your block with the view file path. Ruby on Rails views RSpec.describe The view helper method render produced the view’s rendered HTML, which can be accessed with its corresponding helper method . rendered If you had a view file called index.html.erb that included code, such as , then this is how you can test the assignment to the variable. require 'spec_helper' RSpec.describe 'views/index.html.erb' before do assign(:message, 'great test!') render end it 'should display the message' do expect(rendered).to include 'great test!' end end Learn more Writing effective tests not only will give you confidence in the intended outcome of your tests but also guide you on how it really works, including discovering and ironing out bugs. To find out more about how you can write readable tests that cover the core functionality of Rails, check out ’ other helper methods, for example that cover redirections and aspects of ActiveRecord models. RSpec for Ruby on Rails Read more from ryanwhocodes 5 tips to speed up testing using rspec and ruby on rails Testing private methods in Ruby using Rspec Display your Ruby test coverage using SimpleCov gem and CodeClimate Read more RSpec for Ruby on Rails on Github Rails Controller Testing gem on Github RSpec docs