The add_message method adds a message related to a channel and a user to the messages table:

pub fn add_message(&self, channel_id: Id, user_id: Id, text: &str)
    -> Result<Message, Error>
{
    let timestamp = Utc::now().naive_utc();
    insert_into(messages::table)
        .values((
                messages::timestamp.eq(timestamp),
                messages::channel_id.eq(channel_id),
                messages::user_id.eq(user_id),
                messages::text.eq(text),
                ))
        .returning((
                messages::id,
                messages::timestamp,
                messages::channel_id,
                messages::user_id,
                messages::text,
                ))
        .get_result(&self.conn)
        .map_err(Error::from)
}

The implementation also uses the insert_into function, but we also created the timestamp manually. You can avoid setting this field manually and set a default value to the current timestamp in the timestamp column.