Select columns inside json_agg

I have a query like: SELECT a.id, a.name, json_agg(b.*) as "item" FROM a JOIN b ON b.item_id = a.id GROUP BY a.id, a.name; How can I select the columns in b so I don't have b.item_id in the JSON object? I have read about ROW , but it returns a JSON object like: {"f1": "Foo", "f2": "Bar"} I would need to remap the JSON object once it is fetched to match the proper column keys. I'd like to avoid that and keep original column names.