array_agg DISTINCT and ORDER - postgresql

Question

I'm trying to make a query in PostgreSQL for include results from 2 (or more) tables using left join lateral, and I need to have one record for each record for table entidad_a_ (main table) and all the records from table entidad_b_ must be included in one field generated by array_agg. And in this array, I have to delete duplicate elements and I have to preserve order array in main table.
I need to execute this SQL query:
SELECT entidad_a_._id_ AS "_id", CASE WHEN count(entidadB) > 0 THEN array_agg(DISTINCT entidadB._id,ordinality order by ordinality)
ELSE NULL END AS "entidadB"
FROM entidad_a_ as entidad_a_, unnest(entidad_a_.entidad_b_) WITH ORDINALITY AS u(entidadb_id, ordinality)
LEFT JOIN LATERAL (
SELECT entidad_b_3._id_ AS "_id", entidad_b_3.label_ AS "label"
FROM entidad_b_ as entidad_b_3
WHERE entidad_b_3._id_ = entidadb_id
GROUP BY entidad_b_3._id_
LIMIT 1000 OFFSET 0
) entidadB ON TRUE
GROUP BY entidad_a_._id_
LIMIT 1000 OFFSET 0
But I have errors....
How can I have these results?
Edited:
My error is:
ERROR: function array_agg (integer, bigint) does not exist
SQL state: 42883
Hint: No function matches the given name and argument types. You might need to add explicit type casts.
Character: 69
If the query is:
......array_agg (DISTINCT entidadB._id order by ordinality).....
The eror is:
ERROR: in an aggregate with DISTINCT, ORDER BY expressions must appear in argument list
SQL state: 42P10
Character: 110
My problem is the combination of array_agg, DISTINCT, and ORDER by

Answer 1

Solved!! I've created a postgres extension with a custom aggregation.
CREATE AGGREGATE array_agg_dist (anyelement)
(
sfunc = array_agg_transfn_dist,
stype = internal,
finalfunc = array_agg_finalfn_dist,
finalfunc_extra
);
Creating functions and c code for this custom functions.