How to update part of a JSONField with a computed value?

[Shaheed Haque]

Hi,

I'm using Django 3.2 on Postgres12, and I have a model with a JSONField which contains a simple dict (actually, a Django formset :-)). I would like to update just one value in the dict. I can get this to work when the new value is a hardcoded numeric 333 (quoted, as seems to be needed) like this:

    pay_items.update(inputs=JSONBSet('inputs', ['inputs-0-value'], Value("333"), True))

where JSONBSet is a Func, as indicated below. However, I'd like to actually compute the value. I've tried a variety of things, without success. Here is one attempt:

    pay_items.update(inputs=JSONBSet('inputs', ['inputs-0-value'],
                                     Cast(F('inputs__inputs-0-value'), FloatField()) + Value("3"),
                                     True))

This fails like this:

ERROR:  function jsonb_set(jsonb, unknown, double precision, boolean) does not exist at character 42

HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
STATEMENT:  UPDATE "paiyroll_payitem" SET "inputs" = JSONB_SET("paiyroll_payitem"."inputs", '{inputs-0-value}', (CAST(("paiyroll_payitem"."inputs" -> 'inputs-0-value') AS double precision) + '3'), true) WHERE "paiyroll_payitem"."id" IN (...))

I've tried all sorts of variants with Cast, ExpressionWrapper and so on to no avail, so I'd be grateful for any shove in the right direction!

Thanks, Shaheed

Encls: implementation of JSONBSet derived from https://code.djangoproject.com/ticket/32519 and related threads:

class JSONBSet(Func):
    function = 'JSONB_SET'
    arity = 4
    output_field = CharField()

    def __init__(self, field, path, value, create: bool = True):
        path = Value('{{{0}}}'.format(','.join(path)))
        create = Value(create)
        super().__init__(field, path, value, create)

[Bazil M H]

Use the inbuilt json library in python. Use json.dumps and json.loads

to convert the dictionary to json and vice versa.