use current_user in controller tests

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  
private

  def signed_in?
    !!session[:user_id]
  end
  helper_method :signed_in?

  def current_user=(user)
    session[:user_id] = user.id
  end
  
  def current_user
    return unless signed_in?
    @current_user ||= User.find(session[:user_id])
  end
  helper_method :current_user
end

sessions_controller.rb

class SessionsController < ApplicationController
  def create
    user = User.find_by(email: params[:email])
    
    if user.try(:authenticate, params[:password])
      self.current_user = user
      redirect_to root_url
    else
      flash.now[:notice] = 'Wrong email/password combination'
      render :new
    end
  end
end

sessions_controller_test.rb

require 'test_helper'

class SessionsControllerTest < ActionController::TestCase
  self.use_instantiated_fixtures = true
  
  test "sign in with valid credentials" do
    sign_in @sophie
    
    assert_equal @sophie, current_user
  end
  
  test "sign in with invalid credentials" do
    sign_in @sophie, password: 'wrong'
    
    assert_nil current_user
  end
  
private

  def sign_in(user, password: 'secret')
    post :create, email: user.email, password: password
  end
end

test_helper.rb

ENV["RAILS_ENV"] ||= "test"
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'

class ActiveSupport::TestCase
  ActiveRecord::Migration.check_pending!

  fixtures :all
end

class ActionController::TestCase
  def current_user
    @controller.send(:current_user)
  end
  private :current_user
end

users.yml

sophie:
  email: [email protected]
  password_digest: <%= BCrypt::Password.create('secret') %>