The add_member function requires a channel ID and a user ID to add a membership record to the memberships table:

pub fn add_member(&self, channel_id: Id, user_id: Id)
    -> Result<Membership, Error>
{
    insert_into(memberships::table)
        .values((
                memberships::channel_id.eq(channel_id),
                memberships::user_id.eq(user_id),
                ))
        .returning((
                memberships::id,
                memberships::channel_id,
                memberships::user_id,
                ))
        .get_result(&self.conn)
        .map_err(Error::from)
}

The implementation is simple and it uses the insert_into function call to prepare the INSERT statement to insert a new Membership value in the table. We also need a function to add new messages to a channel.