Dear list,
I was wondering whether there's a package or pattern to define annotations
as model fields, so that they could really be used as database-side
computed fields.
Currently, I do something like this (not tested, it's a simplified case):
class SalesManager(models.Manager):
def get_queryset(self):
return super().get_queryset().annotate(
profit=F("selling_price")-F("buying_price")
)
class Sales(models.Model):
objects = SalesManager()
buying_price = models.FloatField()
selling_price = models.FloatField()
...
Ideally, I'd see something like :
class Sales(models.Model):
buying_price = models.FloatField()
selling_price = models.FloatField()
profit = models.AnnotationField(F("selling_price")-F("buying_price"))
...
An AnnotationField would behave like a read-only field, excepted that it
wouldn't have a database column.
This would have the following benefits :
- much less verbose
- all instance related stuff in one place
- it would make it possible to refer to "profit" as a model field
- you could use the field in Meta.ordering
- you could easily display in django.admin without additional methods
Of course similar result can be achieved with @property and a python
method, but I really need this to be on the database side to use
subqueries, sorting and filtering, etc.
Does anything like this exist ? Is it possible to emulate this behaviour
somehow ?
Thanks !!
Olivier
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