On 05/15/2017 09:32 AM, Zsolt Ero wrote:
In PostgreSQL 9.5+ it is finally possible to modify a single key inside
a JSONB column. Usage is something like this:
update maps set screenshots=jsonb_set(screenshots, '{key}', '"value"')
where id = 10688
Is it possible to write this query using the ORM somehow? If not, please
take it as a feature request.
You can use that function directly:
my_object = session.query(Maps).get(5)
my_object.screenshots = func.jsonb_set(my_object.screenshots, '{key}',
'"value"')
session.flush()
as far as "transparent" ORM use of that, like this:
my_object.screenshots[key] = "value"
right now that is a mutation of the value, and assuming you were using
MutableDict to detect this as an ORM change event, the ORM considers
"screenshots" to be a single value that would be the target of an
UPDATE, meaning the whole JSON dictionary is passed into the UPDATE.
There is no infrastructure for the ORM to automatically turn certain
column updates into finely-detailed SQL function calls. I can imagine
that there might be some event-based way to make this happen
transparently within the flush, however, but I'd need to spend some time
poking around to work out how that might work.
I'm not familiar with what the advantage to jsonb_set() would be and I
can only guess it's some kind of performance advantage. I'd be curious
to see under what scenarios being able to set one element of the JSON
vs. UPDATEing the whole thing is a performance advantage significant
compared to the usual overhead of the ORM flush process; that is,
Postgresql is really fast, and for this optimization to be significant,
you probably need to be calling the Core function directly anyway rather
than going through the whole ORM flush process. But this is all based
on my assumption as to what your goal of using this function is.
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