Django query to retrieve rows that match aggregate criteria in the corresponding table

I am really stuck on a Django request and hope you have a couple of minutes to help me figure it out.

I have a very simple model:

class Task(models.Model):
    # a tuple representing a specific item to be searched for a specific URL
    instructions = models.TextField()

ASSIGNMENT_STATUS_CHOICES = (
    ( 'a', 'assigned' ),
    ( 'c', 'submitted for review' ),
    ( 'f', 'finished' ),
    ( 'r', 'rejected' ),
)

class Assignment(models.Model):
    # the overall container representing a collection of terms for a page found
    # by a user
    user = models.ForeignKey(User)
    task = models.ForeignKey(Task, related_name='assignments')
    status = models.CharField(max_length=1, choices=ASSIGNMENT_STATUS_CHOICES, default='a')

      

        
        
        
      

    

What I want to do is randomly select a task that has less than N assignments, which are the status! = 'R'. In other words, I want to make sure each task completes successfully N times, so if a worker requests a job, she needs one that has fewer than N tasks in a state that could lead to completion.

I was just completely lost trying to find a query that would return tasks like this. For this task, I can check:

task.assignments.exclude(status='r').count() < N

      

        
        
        
      

    

and if so, then this is the candidate. But how can I query Task.objects in such a way that it returns all candidates in one database query so that I can randomly select one of them:

Task.objects.<some magic filter>.order_by('?')[0]

      

        
        
        
      

    

Any help would be appreciated!