Ok, I see not everyone are happy with idea of ORM expression calculated on
Python side:)
We can:
* Provide two different backends (Database-based and Python-based or "run
python in database" backend) for calculated field
* Or provide python-based backend as third party lib
* Or provide whole solution as third party lib (if Django API is flexible
enough for that)
I still do not like database approach.
* If I have list of objects, do you want to have O(n) queries?
* If calculated field is function of only model fields (no foreign keys
etc) it should work locally: some projects may have no database at all.
>>Even simple comparisons can be difficult to simulate in Python. As an
example, some databases treat empty strings as null
But ORM should handle it somehow, should not it?
I want to belive that "filter(alias__isnull=True)" works identically across
all databases.
So, we need to do same on Python side.
>>To be honest, I'm curious why the SQLAlchemy approach isn't being
discussed more here.
I think SQLAlchemy approach is good, but we should use Django ORM
expressions language.
See: we need some high-level expression language to describe functions of
models that could be
calculated both on SQL side and Python side.
And we already have it: it is Django ORM expressions language:)
We have some kind of AST for all those Q, F expressions etc, so we only
need to create evaluator for this language.
No need to use external language, based on Python, as SQLAlchemy does.
Why do we need
from sqlalchemy import func
func.concat(cls.first_name, ' ', cls.last_name)
if we already have
F('first_name') + F('last_name')
?
>>and supplying both the SQL side (via a method which returns a Q object,
most likely) and the Python side
But it ruin DRY idea by making author to write same logic twice (for Python
and SQL), is not it?
Still, user may have ability to provide custom expression,
but for simple cases like "age__gt=18" we can do it "automagically".
Django generates SQL code from its expressions. Sometimes this code is
sophisticated.
Why can't we do that for Python, if we do it for SQL?
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