The default parseNs(), defined in the base class Command, assumes the first element in the cmdObj holds the collection name. If that element is not a string, it assumes there's no collection. This works for commands of the form {find:"foo"}:

string Command::parseNs(const string& dbname, const BSONObj& cmdObj) const {
    BSONElement first = cmdObj.firstElement();
    if (first.type() != mongo::String)
        return dbname;

    return str::stream() << dbname << '.' << cmdObj.firstElement().valueStringData();
}

But explain commands, for example, which use the format {explain:{find:"foo"}}, should override parseNs().

Assigning to backlog-server-platform, but feel free to pass this to a better home if there is one.