Function payload_from_row 

pub fn payload_from_row<T: DeserializeOwned>(
    row: &Value,
    fields: &[&str],
) -> ServiceResult<T>

The payload of a declared event, read out of the row the write stored: the fields listed, by their serialized names, deserialized into the event’s struct. A listed field the row lacks is left out, so an optional one reads as None and a required one is an error — the generator only lists fields the aggregate has.