I handle that situation quite differently.
I have a model manager that annotates different fields to the queryset
depending on when I need them.
For your example, I would have something like this:
class CustomerQuerySet(models.QuerySet):
def with_allowed_to_drink(self):
return self.annotate(allowed_to_drink=Case(When(age__gte=18, then=True),
default=False))
Then I actually created a decorator that applies the manager to instances on a
model. It is not as generic as it could be since it depends on function names
having the word “with” in them, but I’m sure we could make it more general.
def queryset_on_instance(model: Model):
"""Takes the queryset of a model and creates new properties for each
instance of the model based on
the custom queryset class. Some code is copied from
django.models.managers; most of this was inspired by it.
The queryset methods that are copied must have 'with' in their name.
Tip: Use this as a decorator on model classes
"""
def create_method(method_name):
def the_method(self):
return getattr(model._default_manager,
method_name)().get(pk=self.pk)
return the_method
queryset_class = getattr(model._default_manager, "_queryset_class", None)
predicate = inspect.isfunction
for name, method in inspect.getmembers(queryset_class, predicate=predicate):
if 'with' in name:
# Only copy missing attributes.
if hasattr(model, name):
continue
# Only copy public methods or methods with the attribute
`queryset_only=False`.
queryset_only = getattr(method, 'queryset_only', None)
if queryset_only or (queryset_only is None and
name.startswith('_')):
continue
# Copy the method onto the model.
setattr(model, name, create_method(name))
return model
And then on your Customer model:
@queryset_on_instance
class Customer(models.Model):
age = models.IntegerField()
objects = CustomerQuerySet.as_manager()
In your code or template, you would access the value like so:
customer.with_allowed_to_drink.allowed_to_drink
And you can filter by the value like so:
Customers.objects.with_allowed_to_drink().filter(allowed_to_drink=True)
I do agree that it would be nice to have something out of the box that could
handle custom properties in the QuerySet calls, or, as you put it, calculated
fields.
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, November 15, 2017 7:21 AM
To: Django developers (Contributions to Django itself)
Subject: models.CalculatedField feature
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<http://www.jetbrains.com/>
The Drive to Develop
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