I don't know how to make a 1:N relationship on django's db (models.py)

class Subject(models.Model):
        subject_name = models.CharField(max_length=20)

        def __str__(self):
                return self.subject_name


class Professor(models.Model):
        professor_name = models.CharField(max_length=10)
        semester = models.Charfield(max_length=15)
        subject = models.ForeignKey(Subject)

        def __str__(self):
                return self.professor_name


class Board(models.Model):

        professor = models.ForeignKey(Professor):

        def __str__(self):
                return self.professor.professor_name