Populating default value from another table - django

Question

At a bit of a loss here. Basically I have a Product page in the admin section and at the bottom of that I have an TabularInline that let's me choose sizes for the product and set a price. This works but I'm trying to get it so that it pulls the default price from the Sizes table to be the initial value.
I tried the following:
class Size(models.Model):
name = models.CharField(max_length=50)
default_price = models.DecimalField(max_digits=9, decimal_places=2)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now_add=True)
class Meta:
db_table = 'sizes'
ordering = ['-created_at']
verbose_name_plural = 'Sizes'
def __unicode__(self):
return self.name
def default_price(self):
return self.default_price
And then
class ProductOption(models.Model):
product = models.ForeignKey(Product)
size = models.ForeignKey(Size)
price = models.DecimalField(max_digits=9, decimal_places=2, default=Size.default_price)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now_add=True)
It validates, but when I load the admin page I get the following:
unbound method get_default_price() must be called with Size instance as first argument (got nothing instead)
Not sure where else to go from here. I've been looking over the Django docs forever trying to find something else to try. Any ideas?

Answer 1

So, you can override the save method, and if there is no self.id, you can populate some other fields first, that's what sticks out at me as the best solution.
just as an example:
def save(*args, **kwargs):
if not self.id:
self.size = SomeOtherModel.objects.get(pk=size)
super(Size, self).save(*args, **kwargs)

Answer 2

You have several issues here:
Size.default_price is a callable object, so python is trying to call it when setting the default value of your ProductOption. What you might be looking for is:
class Size(models.Model):
# Other stuff here
#property
def default_price(self):
return self.default_price
However, this then causes another, deeper error. If you had the following:
size = Size()
size.default_price
You would wind up with a StackOverflow, due to infinite recursion. This property calls itself. A solution is to have the property name different to the attribute, or in this simplest case, it already acts a bit like a property. You can just remove the whole default_price definition.
However, you are calling Size.default_price, which is not bound to anything if you call it on Size, a class, not on an instance of that class.
What you want to actually do is kind of what Issac Kelly suggests, but more so.
class ProductOption(models.Model):
# column definitions
def __init__(self, *args, **kwargs):
super(ProductOption, self).__init__(*args, **kwargs)
if self.size and not self.price:
self.price = self.size.default_price
This means that when you instantiate a ProductOption, it will automatically set the price based on the size, if there isn't already a price.
You might also want to do this check at save time, as otherwise:
po = ProductOption()
po.size = Size.objects.get(pk=1)
po.save()
# po.price is not set now!
po = ProductOption.objects.get(pk=po.pk)
# po.price is set now!
So, you could have the following:
class ProductOption(models.Model):
# columns
def __init__(self, *args, **kwargs):
super(ProductOption, self).__init__(*args, **kwargs)
self.set_default_price()
def save(self, *args, **kwargs):
self.set_default_price()
super(ProductOption, self).save(*args, **kwargs)
def set_default_price(self):
if self.size and not self.price:
self.price = self.size.default_price
Note the difference in order of the super() call: when initialising the object, we want to call the parent's __init__ first, but when saving, we want to (possibly) set the default value first, then call the parent class's save method.