Hello.
I think I just invented very useful feature.
I have the following model
class Customer(models.Model):
age = models.IntegerField()
I want to show special link in template for customer if she is allowed to
drink.
class Customer(models.Model):
age = models.IntegerField()
@property
def allowed_to_drink(self):
return self.age >= 18
And in template I can do
{% if customer.allowed_to_drink %}.
But in another view I want to show only those customers, who are allowed to
drink.
So, in view I write
drinkers = [customer for customer in models.Customer.objects.all() if
customer.allowed_to_drink].
But what if I have one million customers? It is insane to check this field
on Python side instead of database side.
I use ORM:
models.Customer.objects.filter(age__gte=18).all()
And DRY principle is now violated.
See, I have knowledge "allowed_to_drink -> (age >= 18) " and I want to
write in once, and then use both in ORM queries and object method.
And I am not alone here:
https://stackoverflow.com/questions/31658793/django-filter-query-on-with-property-fields-automatically-calculated
https://stackoverflow.com/questions/2143438/is-it-possible-to-reference-a-property-using-djangos-queryset-values-list
Here is what I suggest
class Customer(models.Model):
age = models.IntegerField()
allowed_to_drink = models.CalculatedField(Q(age__gte=18))
# Using in ORM
Customer.objects.filter(allowed_to_drink=True)
# It actually converted to
Customer.objects.filter(Q(age__gte=18))
# Using it in code
Customer().allowed_to_drink
# This part a little bit tricky, since you will need to check in on Python
side.
# "age__gte" should be converted to
funcs = {
"gte": lambda instance, field, value: getattr(instance, field) >= value
}
instance_field, func = "age__gte".split("__")
funcs[func](instance, instance_field, 18)
But we already have code to convert Q to SQL, so it should be easy to do it
for python.
Some more ideas:
* "Q" may have lazy evalutation here for cases like
"models.CalculatedField(Q(date__gte=now()))". Could be implemented with
lambdas, or we can simply allow only literal here.
* With Postrges you can even store calculated field in database (it has
calculated column), but it works only for literals.
I think this functionality should be supported by Django core because it
affects admin, forms (calculated fields can't be changed), migrations and
ORM.
*Ilya Kazakevich*
PyCharm developer.
http://www.jetbrains.com
The Drive to Develop
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