Django max_length of another model attribute

Question

I want to have the integer value of one model attribute be the max_length of another model attribute like mentioned below "capacity = models.IntegerField(max_length=Concerthall.capacity)".

class Concerthall(models.Model):
    name = models.TextField(max_length=254)
    capacity = models.IntegerField()
    employees = models.IntegerField()

    def __str__(self):
    return self.name

class Events(models.Model):
    name = models.TextField(max_length=254)
    capacity = models.IntegerField(max_length=Concethall.capacity)
    timeFrom = models.DateTimeField()
    timeTo = models.DateTimeField()
    concerthallName = models.ForeignKey(Concerthall, on_delete=models.PROTECT, null=True)

Maybe it is also working with validators but I searched for several hours and wasn't able to find any solution to this.

Answer 1

I suggest a different approach, doing the validation in the models clean() method:

class Events(models.Model):
    name = models.TextField(max_length=254)
    capacity = models.IntegerField(default=0)
    timeFrom = models.DateTimeField()
    timeTo = models.DateTimeField()
    concert_hall = models.ForeignKey(Concerthall, on_delete=models.PROTECT)

    def clean(self):
        if self.capacity > self.concert_hall.capacity:
            raise ValidationError(
                'the capacity of the event cannot exceed the capacity of the hall')