Function presets 

pub fn presets(
    caller: &Caller<'_>,
    names: &NameOverrides,
    schema: &str,
    table: &str,
    verb: Verb,
) -> Result<Vec<(String, Value)>, Fault>

The columns the server fills in itself, with their values.

This is how author_id comes from the caller’s identity rather than from the caller: a preset overrides whatever the request said, so a client that names the column anyway does not get to choose it.