title: Change Default Functionality Of 'One' Model In One-To-Many Relationship In Django Admin
body: <p>I have a one-to-many relationship.</p>
  <pre><code>class User(AbstractUser):
      user_type = models.CharField(choices=USER_TYPES, max_length=255, default='student')
  
  class Teacher(models.Model):
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      
  class Student(models.Model):
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      teacher = models.ForeignKey(Teacher, on_delete=models.CASCADE, related_name='students', default=None, null=True)
  </code></pre>
  <p>When I go into Student in Django Admin I have a dropdown box and can select their Teacher. This is good. However, when I go into Teacher, instead of a dropbox of Students, I get an 'Add Another User' button with a dropdown list of all Users. Clicking a student and pressing 'Save' throws the following error</p>
  <pre><code>Student with this User already exists.
  </code></pre>
  <p>How can I remove 'Add Another User' from the Teacher page and replace it with a dropdown list of all Students (or Students who havent been assigned a Teacher)?</p>
  <p>Thank you.</p>