title: DDD - Aggregate with large relation that is part of invariant
body: <p>Say I have two models</p>
  <p><code>Project</code> and <code>Task</code></p>
  <p>A project can have many tasks, but a project can only have one task with a status of <code>in_progress</code> at any time.</p>
  <p>As far as I understand it, this is normally handled as part of a root aggregate's domain invariant</p>
  <p>In this case however, the list of tasks associated with a project is potentially unbounded, so it doesn't really fit well in my opinion, but at the same time the project should control and maintain the aforementioned invariant</p>
  <p>Any suggestions on how best to model this?</p>