docs: 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