what kind of fields in django models code example

Example 1: django __str__ self multiple

class MyclubUser(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    email = models.EmailField('User Email')

    def __str__(self):
        return self.first_name + " " + self.last_name


class Subscriber(MyclubUser):
    date_joined = models.DateTimeField()

Example 2: primary key django model

class Garden(models.Model):
    garden_id = models.IntegerField(primary_key=True)