title: Accessing field from OneToOne field in ModelForm
body: <p>I am trying to extend Djangos authentication system and have a problem when trying to create the modelForm for it. As you can see below I have referenced the authentication backend via the suggested OneToOnefield however when I am creating the ModelForm if I try to reference the fields such as 'username', 'password' etc. it spits an error saying they are unknown fields. The form I am creating is a registration from. What am I doing wrong here? Cheers</p>
  
  <p>Model -</p>
  
  <pre><code>class StudentModel(models.Model):
      user = models.OneToOneField(User, unique=True)
      birth_date = models.DateField()
      contact_number = models.IntegerField()
      referral = models.CharField(max_length=100, choices=referral_choices)
  </code></pre>
  
  <p>ModelForm - </p>
  
  <pre><code>from django import forms
  from opus_login.models import StudentModel, EmployerModel
  
  class StudentForm(forms.ModelForm):
  
      class Meta:
          model = StudentModel
          fields = ['username', 'first_name']
  </code></pre>
  
  <p>Error -</p>
  
  <pre><code>django.core.exceptions.FieldError: Unknown field(s) (username, first_name) specified for StudentModel
  </code></pre>