Selecting (geo)json from postgis

Postgis does not support selecting feature collections, thus I had to build one on my own. This is the resulting query I used:

SELECT json_build_object(
    'type', 'FeatureCollection',
    'properties', json_build_object(),
    'features', json_agg(st_asgeojson(field.geometry)::json)
           )
FROM field
WHERE farm_id = 'XYZ';

Useful postgis functions here are json_build_object() which creates a new object and json_agg() which creates a new array. Other json operations can be found in the postgres documentation.