title: list oldest employee for each department
body: <p>I have those two models. I need to return the list of oldest employee for each department.</p>
  
  <pre><code>class Employee(models.Model):
      name = models.CharField(max_length=50)
      birth_day = models.DateField()
      department = models.ForeignKey('Department')
  
  class Department(models.Model):
      sector = models.CharField(max_length=255)
  </code></pre>
  
  <p>I'm a bit stuck about how to make it in Django.</p>