users_controller.rb

class UsersController < ApplicationController
  def new
    @user = User.new
  end

  def create
    user = User.new(params[:user])
    user.save # breaks the: "creates a new user with the given attributes" with error: Mock "User_1001" received unexpected message :save with (no args)
    render :nothing => true
  end
end

users_controller_spec.rb

require 'spec_helper'

describe UsersController do

  let(:user) { mock_model(User) }

  let(:user_attrs) do
    {"name" => "testos", 
     "email" => "testos@teroni.fi", 
     "password" => "secretos" }
  end

  describe "GET new" do
    it "assigns new user" do
      get :new
      assigns[:user].should be_a_new User
    end

    it "renders the new user template" do
      get :new
      response.should render_template :new
    end
  end

  describe "POST create" do
    before do
      User.stub(:new).and_return(user)
    end

    it "creates a new user with the given attributes" do
      User.should_receive(:new).with(user_attrs).and_return(user)
      post :create, :user => user_attrs
    end

    it "saves the user" do
      user.should_receive(:save)
      post :create
    end

  end

end