Help with implementing calculated fields (#28822)

[Tobias Bengfort]

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

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

[schinckel]

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.

[Tobias Bengfort]

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.

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