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
      setup do
        @post = Factory.build(:post)
        Post.stubs(:new).returns(@post)
      end

      should "create a new post with params" do
        params = {'some' => 'params'}
        Post.expects(:new).with(params).returns(post)
        post :create, :post => params
      end

      should "assign the new post" do
        post :create
        assigns(:post).should == @post
      end

      should "try to save the new post" do
        @post.expects(:save)
        post :create
      end

      context "with valid input" do
        setup do
          @post.stubs(:save).returns(true)
          post :create
        end

        should_redirect_to("post index") { posts_path }
      end

      context "with invalid input" do
        setup do
          @post.stubs(:save).returns(false)
          post :create
        end

        should_respond_with :success
        should_render_template :new
      end
    end  
  end
end