title: MongoMapper: how do I create a model like this
body: <p>Suppose we have two models, Task and User.
  So a user can have many tasks and tasks should be able to have many users too. But, a task should also have a unique creator who is also a user.</p>
  
  <p>Exemple:</p>
  
  <p>A task in this context is like this:
  Task ID, Task Creator, Users who should do the task</p>
  
  <p>User_1 creates a task and he is then the creator.
  User_1 specifies User_2 and User_3 as users who should do the task. So these two last users are not creators of task.</p>
  
  <p>How do I create this models so that if I have a task object, I can find it's creator and users who should complete it. And how do I do, if I have a user, to find all tasks he created and all tasks he should complete.</p>