django relations 2

GRADE_CHOICES = (
    ("A", "A"),
    ("B", "B"),
    ("C", "C"),
    ("D", "D"),
    ("F", "F"),
)

class Lecturer(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)

    def __str__(self):
        return f"{self.first_name} {self.last_name}"


class Subject(models.Model):
    name = models.CharField(max_length=100)
    code = models.CharField(max_length=10)
    lecturer = models.ForeignKey("Lecturer", on_delete=models.SET_NULL, null=True)

    def __str__(self):
        return f"{self.name}"


class Student(models.Model):
    student_id = models.CharField(max_length=10, primary_key=True)
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    birth_date = models.DateField()
    email = models.EmailField(unique=True)
    subjects = models.ManyToManyField(to='Subject',
                                      through='StudentEnrollment')

class StudentEnrollment(models.Model):
    student = models.ForeignKey(to='Student',
                                on_delete=models.CASCADE)
    subject = models.ForeignKey(to='Subject',
                                on_delete=models.CASCADE)
    enrollment_date = models.DateField(default=date.today)
    grade = models.CharField(max_length=1, choices=GRADE_CHOICES,
                             blank=True, null=True)