Use the inbuilt *json* library in python. Use json.dumps and json.loads
to convert the dictionary to json and vice versa.
On Thursday, 5 August 2021 at 04:17:38 UTC+5:30 shahee...@gmail.com wrote:
> 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)
>
>
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