Hi Matt,
sorry for the late reply. This is exactly what I was looking for!
However, if I understand correctly this is more a proof of concept and
not ready for production use. There still seem to be some subtlties that
need to be fixed:
- related queries with shared_property
- filtered aggregates (e.g. Max('name', filter=Q(…)) in related queries
with ComputedField (Q object would need similar processing to F objects)
- group_by with both implementations (I get "aggregate functions are not
allowed in the GROUP BY clause")
Other than that it is great to see this is possible as a third party
library. I am still not sure how much interest there actually is though.
tobias
PS: There is a small typo in the first link in your blog post.
On 15/03/2021 04.49, schinckel wrote:
Hi Tobias.
I've done a bit of stuff on this, and found there were actually limited
changes that need to be made to django to got this to work.
https://schinckel.net/2020/09/15/django-properties-from-expressions%2C-or-computedfield-part-2/
I haven't touched it much since then (although I'm keen to revisit it,
actually), but it's actually possible that we can do it without any
changes to Django.
https://github.com/schinckel/django-shared-property
Matt.
On Sunday, March 14, 2021 at 11:46:15 PM UTC+10:30 Tobias Bengfort wrote:
Hi,
a while back there has been a discussion about calculated model fields:
https://groups.google.com/g/django-developers/c/ADSuUUuZp3Q/m/5o02Lp_jCwAJ
<https://groups.google.com/g/django-developers/c/ADSuUUuZp3Q/m/5o02Lp_jCwAJ>
The main benefit would be that these would be available in cases where
annotations are currently not available, e.g. related queries or admin
lists (see #14336).
Note that the topic of fields that have both a DB and python
implementation was discussed and the consensus was that the python
equivalents could be added by a separate library, if required. So I am
only talking about the database part here.
The linked discussion was mainly about *what* is desired, not *how* it
could be done. I also noticed that the fellows did not participate in
that discussion.
I would like to have a shot at this topic. So I am interested in:
- Do you think this is even possible/worth the effort?
- How could this be implemented and what would be potential challenges?
- Is it possible to implement this in a third party library or does it
require changes to django itself?
I would probably start with a non-editable, non-concrete model field
(similar to GenericForeignKey or ForeignObjRel). But I have no clue yet
how to integrate that with QuerySet/Query.
thanks
tobias
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