Return a find command document for this query.

        Should be called *after* get_message.

def as_command(self):
        """Return a find command document for this query.

        Should be called *after* get_message.
        """
        if '$explain' in self.spec:
            self.name = 'explain'
            return _gen_explain_command(
                self.coll, self.spec, self.fields, self.ntoskip,
                self.limit, self.batch_size, self.flags,
                self.read_concern), self.db
        return _gen_find_command(
            self.coll, self.spec, self.fields, self.ntoskip, self.limit,
            self.batch_size, self.flags, self.read_concern,
            self.collation), self.db