Convert list [Task (username, description)] to Map [username, Set [Task]]

(NOTE: I stop working with Scala and continue to struggle with the most common collection manipulation operations.)

I would like to convert a List [Task] to a Map. Here are some details:

// assignee may be null
case class Task(assignee: String, description: String)
// might refactor it into:
// case class Task(assignee: Option[String], description: String)

      

        
        
        
      

    

I want a Map where Keys are assignees and each value is a Set [Task]. I am having trouble managing the following two situations:

• The map is not (cough) friendly (cough) with null keys (I worked on this with the [String] option for assignee) and
• need to distinguish if the key exists in the map (just add the value to the existing set) and the key has already been added, so the Set value exists

I came up with the following, but it looks too verbose.

def groupByAssignee(tasks : List[Task]) : Map[Option[String], Set[Task]] = {
 tasks.foldLeft(Map[Option[String], Set[Task]]())(
  (m, t) => {
    m.get(t.assignee) match {
      case Some(_) => m + ((t.assignee, m.get(t.assignee).get.+(t)))
      case _       => m + ((t.assignee, Set(t)))
    }
  })
}

      

        
        
        
      

    

What's an easier / cleaner way to achieve this?

Thank!