sessions_controller.rb

class SessionsController < ApplicationController
  skip_before_filter :login_required
  def create
    user = User.authenticate(params[:username], params[:password])
    if user
      session[:user_id] = user.id
      flash[:notice] = "Logged in successfully."
      redirect_to_target_or_default(twikets_path)
    else
      flash.now[:error] = "Invalid username or password!"
      render :action => 'new'
    end
  end
end

sessions_spec.rb

require 'spec_helper'

describe SessionsController do
  let(:user) { User.make(:username => 'bob') }

  describe '#create' do
    context 'authenticate fail' do
      before(:each) { User.stubs(:authenticate).returns(nil) }

      it 'session[:user_id] should == user.id' do
        post :create
        session[:user_id].should be_nil
      end

      it 'should render_template :new' do
        post :create
        response.should render_template :new
      end

      #############
      # Failing Spec
      ##################################################################
      it 'flash.now[:error] should be set to failure' do
        post :create
        flash.now[:error].should == "Invalid username or password!"
      end
      ##################################################################

    end

    context 'authenticate successfully' do
      before(:each) { User.stubs(:authenticate).returns(user) }

      #############
      # Failing Spec
      ##################################################################
      it 'should redirect' do
        expects(:redirect_to_target_or_default).with(twikets_path)
        post :create
      end
      ##################################################################

      it 'session[:user_id] should == user.id' do
        post :create
        session[:user_id].should == user.id
      end

      it 'flash[:notice] should be set to success' do
        post :create
        flash[:notice].should == "Logged in successfully."
      end
    end
  end
end

spec_helper.rb

# This file is copied to ~/spec when you run 'ruby script/generate rspec'
# from the project root directory.
ENV["RAILS_ENV"] ||= 'test'
require File.dirname(__FILE__) + "/../config/environment" unless defined?(Rails)
require 'rspec/rails'
# Requires supporting files with custom matchers and macros, etc,
# in ./support/ and its subdirectories.
Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each {|f| require f}

Remarkable.include_matchers!(Remarkable::ActiveRecord, Rspec::Core::ExampleGroup)
Remarkable.include_matchers!(Remarkable::ActiveModel, Rspec::Core::ExampleGroup)

Rspec.configure do |config|
  # == Mock Framework
  #
  # If you prefer to use mocha, flexmock or RR, uncomment the appropriate line:
  #
  config.mock_with :mocha
  # config.mock_with :flexmock
  # config.mock_with :rr
  # config.mock_with :rspec

  # If you'd prefer not to run each of your examples within a transaction,
  # uncomment the following line.
  # config.use_transactional_examples = false
end