PostgreSQL Function: Return Multiple Rows as JSON - json

Question

I have a PostgreSQL function which successfully returns its data as
table(key character varying, value integer)
but I want to return JSON as it's more convenitent. I have looked at some other answers, and documentation, but it gets complicated when aliases are used (I'd like to return 'key' and 'value' as the column names).
Can someone please recommend the most simple and concise way of expressing this function, in a way that does not require extra complexity to call:
DROP FUNCTION get_document_metadata_integer_records_by_document_id(integer);
CREATE OR REPLACE FUNCTION get_document_metadata_integer_records_by_document_id(document_id integer)
RETURNS table(key character varying, value integer) AS
$BODY$
SELECT document_metadata_records.key, document_metadata_integer_records.value FROM document_metadata_integer_records
LEFT OUTER JOIN document_metadata_records
ON document_metadata_integer_records.document_metadata_record_id = document_metadata_records.id
WHERE document_metadata_records.document_id = $1
$BODY$
LANGUAGE sql VOLATILE
COST 100;
ALTER FUNCTION get_document_metadata_integer_records_by_document_id(integer)
OWNER TO postgres;

Answer 1

Use cross join lateral as a clean and easy path to create a composite type that can be used by row_to_json
create or replace function get_document_metadata_integer_records_by_document_id(
_document_id integer
) returns setof json as $body$
select row_to_json(s)
from
document_metadata_integer_records dmir
left outer join
document_metadata_records dmr on dmir.document_metadata_record_id = dmr.id
cross join lateral
(select dmr.key, dmir.value) s
where dmr.document_id = _document_id
;
$body$ language sql stable;
then use it as
select get_document_metadata_integer_records_by_document_id(1) as pair;
or
select pair from get_document_metadata_integer_records_by_document_id(1) j(pair);
or make it return a table
) returns table (pair json) as $body$
and use as
select pair from get_document_metadata_integer_records_by_document_id(1);
With older versions of Postgresql without lateral it is possible to do it in a subquery
select row_to_json(s)
from
(
select dmr.key, dmir.value
from
document_metadata_integer_records dmir
left outer join
document_metadata_records dmr on dmir.document_metadata_record_id = dmr.id
where dmr.document_id = _document_id
) s