Annotations as models fields

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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