How to use the result of a graphene resolved field in another field

Question

I have this use case:

class ProjectType(graphene.objectType):

  tasks = graphene.List(TaskType)
  duration = graphene.Int() # days

  def resolve_tasks():
    return self.tasks.all()
  
  def resolve_duration():
    return get_duration_from_tasks(self.tasks.all())

A project can have many tasks, so self.tasks.all() can be an expensive db query to do twice.