Django. Default value from foreign key

Question

I use Django 1.9.1, Python 3.5. models.py :

class Item(models.Model):
    name = models.CharField(max_length=200)
    price = models.FloatField()
    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Lot(models.Model):
    item = models.ForeignKey(Item)
    count = models.IntegerField(default = 1)
    price = models.FloatField(default = 1) #Price on the moment of buying
    def __str__(self):              # __unicode__ on Python 2
        return self.item.name

    def cost(self):
         return self.price * self.count

I want to create Lot object wiht default price = item.price . Ie price on the moment of buying. So I can't obtain price value from Lot.item.price because it may be different. When code of models.py is like that:

class Lot(models.Model):
    item = models.ForeignKey(Item)
    count = models.IntegerField(default = 1)
    price = models.FloatField(default = item.price) #Price on the moment of buying
    def __str__(self):              # __unicode__ on Python 2
        return self.item.name

    def cost(self):
         return self.price * self.count

I get following error:

AttributeError: 'ForeignKey' object has no attribute 'price'

How should I correct this code?

Answer 1

default in your model definition is not 'instance-aware'. I'd suggest overriding the save method of Lot to pull in price at time of save.

class Lot(models.Model):

    item = models.ForeignKey(Item)
    count = models.IntegerField(default = 1)
    price = models.FloatField(default = item.price) #Price on the moment of buying
    def __str__(self):              # __unicode__ on Python 2
        return self.item.name

    def save(self, *args, **kwargs):
        if self.item: # verify there's a FK
            self.price = self.item.price
        super(Lot, self).save(*args,**kwargs) # invoke the inherited save method; price will now be save if item is not null

    def cost(self):
         return self.price * self.count

Answer 2

You should override Lot.save to set the default value for price.

class Lot(models.Model):
    item = models.ForeignKey(Item)
    price = models.FloatField()
    ....

    def save(self, *args, **kwargs):
        if not self.price:
            self.price = self.item.price
        super(Lot, self).save(*args, **kwargs)