  # cases.
  
  
  class X(models.Model):
      y = models.ForeignKey('Y')
  
  
  class Y(models.Model):
      x1 = models.ForeignKey(X, related_name='y1')
  
 
</s> remove     x1 = models.ForeignKey(X, related_name='y1')
 </s> add     x1 = models.ForeignKey(X, models.CASCADE, related_name='y1') </s> remove     y = models.ForeignKey('LoopY')
 </s> add     y = models.ForeignKey('LoopY', models.CASCADE) </s> remove     parent = models.ForeignKey(SomeParentModel)
 </s> add     parent = models.ForeignKey(SomeParentModel, models.CASCADE) </s> remove     content_type = models.ForeignKey(ContentType)
 </s> add     content_type = models.ForeignKey(ContentType, models.CASCADE) </s> remove     et = models.ForeignKey(ExtraTerrestrial)
 </s> add     et = models.ForeignKey(ExtraTerrestrial, models.CASCADE)
    y = models.ForeignKey('Y', models.CASCADE)
  # cases.
  
  
  class X(models.Model):
    y = models.ForeignKey('Y', models.CASCADE)
  
  
  class Y(models.Model):
      x1 = models.ForeignKey(X, related_name='y1')
  
keep keep keep keep replace keep keep keep keep keep
 Fixed #21127 -- Started deprecation toward requiring on_delete for ForeignKey/OneToOneField 
tests/queries/models.py