api.v0alpha.FieldIndex(FieldIndex)

first value is the field_name in the record.
additional values are nested fields on record field looked up from index[0]

{
    "datetime": {
        "years": 0,
        "weeks": 0,
        "days": 0,
        "hours": 0,
        "minutes": 0,
        "seconds": 0,
        "duration": "string"
    }
}