django db generated model fields - question and what appears to be a surprising behavior

[alonn]

I'm excited for the newly introduced Django db generated model fields (in Django 5.0) as it opens multiple new options. but I just bumped into a strange behavior.

consider this model

VAT.= 1.17

class Cost(models.Model):

   price = DecimalField() # cut the definition for brevity

   price_with_vat = GeneratedField(expression=F("price")*VAT, db_persist=True, output_field=DecimalField())

but if I do :

cost = Cost.objects.create(price=Decimal(10))

print(cost.price_with_vat) 

# Decimal(0)

but if I do

cost_from_db = Cost.objects.get(price=Decimal(10))

print(cost_from_db.price_with_vat)

# Decimal(11.7)

This is a surprising behavior,  is this expected? am I doing something wrong? 

Also - I try to understand this behavior more deeply and build my mental model of those-  what are the differences between "generated fields" and using the F expressions in the a queryset? My guess, with db_persist it's more like a materialized view, but what other differences exist that I might need to be aware of 

 maybe someone wants to do a talk on that :) 

[Meir Kriheli]

It's a bit like materialized views (or views in general): views over table, generated over column.

I recommended reading the PG docs on it:

https://www.postgresql.org/docs/current/ddl-generated-columns.html

Regarding Django's behavior, I would check the table schema, and log the executed query upon INSERT (specially the RETURNING part). That may provide a good insight.

Cheers

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Meir Kriheli
http://meirkriheli.com