Postgres convert json with duplicate IDs

Question

With this select:

json_agg(json_build_object("id", price::money))

I get the resulting value:

[
  {"6" : "$475.00"}, 
  {"6" : "$1,900.00"},
  {"3" : "$3,110.00"},
  {"3" : "$3,110.00"}
]

I would like the data in this format instead:

{
  "6": ["$475.00","$1,900.00"],
  "3": ["$3,110.00","$3,110.00"]
}

When queried on the server or used with jsonb, the IDs are duplicate and only one of the key value pairs make it through.

Answer 1

You should aggregate prices in groups by ids and use the aggregate function json_object_agg(). You have to use a derived table (subquery in the from clause) because aggregates cannot be nested:

select json_object_agg(id, prices)
from (
    select id, json_agg(price::money) as prices
    from my_table
    group by id
    ) s

Working example in rextester.