title: MVVM model responsibility
body: <p>This is a basic MVVM question.</p>
  
  <p>Lets say that i have a student editor window which allows the user to set the student's payment method (cash or cheque). For flexibility, the possible payment methods must be retrieved from the server and the list varies on the student's age which can also be changed.</p>
  
  <p>The questions are: </p>
  
  <ol>
  <li><p>where should the possible payments methods be stored? Model or view model?</p></li>
  <li><p>if model, when the user changes the age who should download the new list of payment methods?</p></li>
  </ol>
  
  <p>what should the model contain and do in MVVM?</p>