class Book(models.Model):     name = models.CharField(max_length=10)     pub_date = models.DateField()     classification = models.ForeignKey(Classification, on_delete=models.DO_NOTHING, null=True)     student = models.ForeignKey(Student, on_delete=models.DO_NOTHING, null=True)     author = models.CharField(max_length=8)     content = models.CharField(max_length=50)