gistfile1.rb

require 'spec_helper'

describe SharesController do
  render_views

  describe '#create' do
    let(:share_params) { :facebook_id => 'abc', :address_id => 7 }

    it "redirects to login" do
      post :create, :share => share_params
      response.should redirect_to login_path
    end

    context "if the user is logged in" do
      before { @user = login_user }

      context "and the parameters are valid" do
        it "creates a new Share" do
          share = mock_model(Share)
          share.should_receive(:save).and_return(true)
          Share.should_receive(:new).
            with(share_params.merge(:user_id => @user.id)).
            and_return(share)
          post :create, :format => :json, :share => share_params
        end
        
        it "responds with success" do
          Share.stub_chain(:new, :save).and_return(true)
          post :create, :format => :json, :share => share_params
          response.should be_success
        end
      end
      
      context "and the parameters are invalid" do
        # ...
      end
    end

  end
end

Beautiful, thanks for teaching me!