$ gem install simple_command

# define a command class
class AuthenticateUser
  # put SimpleCommand before the class' ancestors chain
  prepend SimpleCommand
  include ActiveModel::Validations

  # optional, initialize the command with some arguments
  def initialize(email, password)
    @email = email
    @password = password
  end

  # mandatory: define a #call method. its return value will be available
  #            through #result
  def call
    if user = User.find_by(email: @email)&.authenticate(@password)
      return user
    else
      errors.add(:base, :failure)
    end
    nil
  end
end

# config/locales/en.yml
en:
  activemodel:
    errors:
      models:
        authenticate_user:
          failure: Wrong email or password

class SessionsController < ApplicationController
  def create
    # initialize and execute the command
    # NOTE: `.call` is a shortcut for `.new(args).call`
    command = AuthenticateUser.call(session_params[:email], session_params[:password])

    # check command outcome
    if command.success?
      # command#result will contain the user instance, if found
      session[:user_token] = command.result.secret_token
      redirect_to root_path
    else
      flash.now[:alert] = t(command.errors.full_messages.to_sentence)
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

describe AuthenticateUser do
  subject(:context) { described_class.call(username, password) }

  describe '.call' do
    context 'when the context is successful' do
      let(:username) { 'correct_user' }
      let(:password) { 'correct_password' }
      
      it 'succeeds' do
        expect(context).to be_success
      end
    end

    context 'when the context is not successful' do
      let(:username) { 'wrong_user' }
      let(:password) { 'wrong_password' }

      it 'fails' do
        expect(context).to be_failure
      end
    end
  end
end