mongodb更新數組中的所有匹配項

假如集合中有如下數據

{ "_id" : ObjectId("4b97e62bf1d8c7152c9ccb74"), "title" : "ABC",
  "comments" : [ { "by" : "joe", "votes" : 3 }, { "by" : "jane", "votes" : 7 } ] }

將comments值中所有votes的值改為1

 t.update( obj, {$set:{'comments.$.votes':1}}, false, true )

結果為:

{ "_id" : ObjectId("4b97e62bf1d8c7152c9ccb74"), "title" : "ABC",
  "comments" : [ { "by" : "joe", "votes" : 1 }, { "by" : "jane", "votes" : 1 } ] }