posts_controller.rb

def PostsController < ActionController::Base
  def create
    @post = Post.new(params[:post])

    if @post.save
      redirect_to posts_path
    else
      render :action => "new"
    end
  end
end

posts_controller_test.rb

class PostsControllerTest < ActionController::TestCase
  context "A PostsController" do
    context "on POST to :create" do
      context "with valid input" do
        setup do
          post :create, :post => Factory.attributes_for(:post)
        end

        should_redirect_to("post index") { posts_path }
        should_change("number of posts", :by => 1) { Post.count }
      end

      context "with invalid input" do
        setup do
          post :create, :post => Factory.attributes_for(:post, :title => "")
        end

        should_respond_with :success
        should_render_template :new
        should_not_change("number of posts") { Post.count }
      end
    end  
  end
end