Re: json accessors

On 11/29/2012 01:10 AM, Merlin Moncure wrote:
> On Wed, Nov 28, 2012 at 2:44 PM, Andrew Dunstan <andrew@dunslane.net> wrote:
...
>
>>> *) have you considered something like
>>> anyelement from_json(anyelement, json)
>>> or
>>> select <json>::some_type;  (this may or many not be possible given our
>>> casting mechanics; i don't know).
>> I have no idea what the semantics of this would be.
> Yeah, there's a lot of nuance there.
One way to tackle it would give the argument element as a template
and the result will the same template filled in from json filled

create table tab1(id serial primary key, ts timestamp default now(), 
data text);

insert into tab1 select from_json(row(null,null,null)::tab1, 
'{"data":"the data"}');
insert into tab1 select from_json(row(null,null,null)::tab1, '{"id":-1, 
"ts":null, "data":""}');
insert into tab1 select from_json(t.*,'{"data":"more data"}') from tab1 
t where id = -1;

hannu=# select row_to_json(t.*) from tab1 t;                          row_to_json
--------------------------------------------------------------- {"id":1,"ts":"2012-11-29 02:01:48.379172","data":"the
data"}{"id":-1,"ts":null, "data":""} {"id":2,"ts":"2012-11-29 02:02:34.600164","data":"more data"}
 
(3 rows)

if extracting the defaults from table def proves too tricky for first 
iteration, then
just set the missing fields to NULL or even better, carry over the 
values from template;

------------------------------
Hannu

PS: good work so far :)

Hannu