How to invoke post method using stubs and mocks in ruby on rails

Question

Im new to this mocking and stubbing in rspec. Actually in my controller, i have an action method invite. I would like to mock/stub this invite method in my contoller.... Below is my code:

when(/^I invite a subject with required attributes$/) do
  patient_enrollment = {subject_enrollment{
    country_code: "USA",
    subject_id: "Subj 100",
    subject_uuid: "23451"}}
  return_enrollment = {subject_enrollment{
    country_code: "USA",
    subject_id: "Subj 100",
    activation_code: "xcvbnh",
    subject_uuid: "23451"}}

patient_enrollment is the parameter which invite action(Through Post Method) accepts and i want it to return return_enrollment...