I have the field 'submission' which has a user and a problem. How can I get an SQL search result which will give a list of only one result per user-problem pair?

Models are like this:

```
class Problem(models.Model):
    title = models.CharField('Title', max_length = 100)
    question = models.TextField('Question')

class Submission(models.Model):
    user = models.ForeignKey(User)
    problem = models.ForeignKey(Problem)
    solution = models.CharKey()
    time = models.DateTimeField('Time', auto_now_add=True)

```