old_contents: from annoying.fields import AutoOneToOneField
  from django.conf import settings
  from django.db import models
  from mentoring.models import Relationship
  
  class Preferences(models.Model):
      """
      Records the mentorship opinions of a User.
  
      """
      class Meta:
          verbose_name_plural = 'Preferences'
  
      user = AutoOneToOneField(
          settings.AUTH_USER_MODEL, on_delete=models.CASCADE,
          related_name='mentorship_preferences',
          primary_key=True)
      is_seeking_mentor = models.BooleanField(default=False)
      is_seeking_mentee = models.BooleanField(default=False)
      mentor_requirements = models.TextField(blank=True)
      mentee_requirements = models.TextField(blank=True)
  
  class Invitation(models.Model):
      """
      An invitation to form a mentoring relationship.
  
      """
      # The possible responses to an invitation.
      ACCEPT = 'A'
      DECLINE = 'D'
      RESPONSES = ((ACCEPT, 'Accept'), (DECLINE, 'Decline'))
  
      mentor = models.ForeignKey(settings.AUTH_USER_MODEL,
                                 related_name='mentor_invitations')
      mentee = models.ForeignKey(settings.AUTH_USER_MODEL,
                                 related_name='mentee_invitations')
  
      created_by = models.ForeignKey(
          settings.AUTH_USER_MODEL, related_name='created_invitations')
      created_on = models.DateField(auto_now_add=True)
  
      mentor_response = models.CharField(max_length=1, choices=RESPONSES)
      mentee_response = models.CharField(max_length=1, choices=RESPONSES)
  
      is_active = models.BooleanField(default=True)
  
      # Note: English spelling, not American
      cancelled_on = models.DateField(blank=True, null=True)
  
      relationship_started = models.ForeignKey(
          Relationship, blank=True, null=True,
          related_name='started_by_invitations')
  
new_contents: from annoying.fields import AutoOneToOneField
  from django.conf import settings
  from django.db import models
  from mentoring.models import Relationship
  
  class Preferences(models.Model):
      """
      Records the mentorship opinions of a User.
  
      """
      class Meta(object):
          verbose_name_plural = 'Preferences'
  
      user = AutoOneToOneField(
          settings.AUTH_USER_MODEL, on_delete=models.CASCADE,
          related_name='mentorship_preferences',
          primary_key=True)
      is_seeking_mentor = models.BooleanField(default=False)
      is_seeking_mentee = models.BooleanField(default=False)
      mentor_requirements = models.TextField(blank=True)
      mentee_requirements = models.TextField(blank=True)
  
  class Invitation(models.Model):
      """
      An invitation to form a mentoring relationship.
  
      """
      # The possible responses to an invitation.
      ACCEPT = 'A'
      DECLINE = 'D'
      RESPONSES = ((ACCEPT, 'Accept'), (DECLINE, 'Decline'))
  
      mentor = models.ForeignKey(settings.AUTH_USER_MODEL,
                                 related_name='mentor_invitations')
      mentee = models.ForeignKey(settings.AUTH_USER_MODEL,
                                 related_name='mentee_invitations')
  
      created_by = models.ForeignKey(
          settings.AUTH_USER_MODEL, related_name='created_invitations')
      created_on = models.DateField(auto_now_add=True)
  
      mentor_response = models.CharField(max_length=1, choices=RESPONSES)
      mentee_response = models.CharField(max_length=1, choices=RESPONSES)
  
      is_active = models.BooleanField(default=True)
  
      # Note: English spelling, not American
      cancelled_on = models.DateField(blank=True, null=True)
  
      relationship_started = models.ForeignKey(
          Relationship, blank=True, null=True,
          related_name='started_by_invitations')
  
subject: fix use of oldstyle class