Question

I am trying to set a default value for attribute threshold in this code, the threshold should be the current level*50 and this is the model

class Level (models.Model):
    name = models.CharField(max_length=20,null=True, blank=True)
    description = models.CharField(max_length=20, null=True, blank=True)
    number = models.IntegerField(null=True, blank=True)
    threshold = models.IntegerField(null=True, blank=True,default=50*number,editable=False)

i get an error unsupported operand types for * : 'int' and 'IntegerField'

Answer 1

You best best is to do such calculation while saving the object. So override Model.save

or a better generic way would be to write a custom field and override pre_save

class DependentIntegerField(models.IntegerField):

    def pre_save(self, model_instance, add):
        if not add: # set the default only while adding model
            return super(self, DependentIntegerField).pre_save(model_instance, add)

        return model_instance.number*50

You can further enhance it and make DependentIntegerField generic so that you can pass callable to it and do any calculation, and you can do further enhancements like checking if user has set the value or not before using default value, and to make it more generic so that you can make any Field as dependent field by passing the field class to a factory function. e.g.

from django.db import models

class_map = {}

def depends_field_pre_save(self, model_instance, add):
    """
    if default is not callable or it is not a model add, lets skip our hook
    """
    if not add or not callable(self.default):
        super(self.__class__, self).__init__(self,*args, **kwargs)
    value = self.default(model_instance)
    setattr(model_instance, self.attname, value)
    return value

def FieldDepends(field_class):
    """
    return a dervied class from field_class which supports dependent default
    """
    if field_class in class_map:
        # we already created this class so return that
        return class_map[field_class]

    new_class = type('Depends'+field_class.__name__, (field_class,), {'pre_save':depends_field_pre_save })

    class_map[field_class] = new_class

    return new_class

and use it like this

class DependentModel(models.Model):

    def threshold_default(model_instance=None):
        if model_instance is None:
            return 10
        return model_instance.number*10

    number = models.IntegerField(null=True, blank=True, default=10)
    threshold = FieldDepends(models.IntegerField)(null=True, blank=True, default=threshold_default,editable=False)

I have created a small django project djangodepends on bitbucket with test cases