mongodb "$"的作用

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{    "_id": ObjectId("58e48e32830f398e8f401a95"),    "_class": "com.ydd.model.mongdb.WeiXinQuestion",    "openid": "oDVO_w2juwMejm7S-UxfLaSOC0kU21",    "createtime": NumberLong(1491373618008),    "endtime": NumberLong(0),    "antotal": NumberLong(2),    "content": "asdfasdfasdfasdfasdf",    "status": 0,    "isshow": 1,    "address": "",    "sex": 0,    "labels": "氣血兩虛",    "besttotal": 0,    "bestscore": 0,    "clerkAnswer": [        {            "_id": "p0k94bZ4EHV9XsftlMlOzHS8L",            "clerkId": 2159,            "headerimg": "head/2159/201612/1480922352820.jpeg",            "name": "藥點點店長7",            "content": "高難度",            "createtime": NumberLong(1491373686025),            "adopt": 0,            "score": 0,            "zan": 0,            "ids": [                            ],            "clerkAnswerScore": [                            ],            "qid": "58e48e32830f398e8f401a95",            "status": 0,            "isReward": 0,            "replayAnswers": [                            ]        },        {            "_id": "GkJtDMB7uizt699qFyEhAINZ0",            "clerkId": 2166,            "headerimg": "head/2166/201701/1484742330434.jpeg",            "name": "藥點店員13",            "content": "但",            "createtime": NumberLong(1491373800122),            "adopt": 0,            "score": 0,            "zan": 0,            "ids": [                            ],            "clerkAnswerScore": [                            ],            "qid": "58e48e32830f398e8f401a95",            "status": 0,            "isReward": 0,            "replayAnswers": [                            ]        },        {            "_id": ObjectId("58b5b0eead349879b2871348"),            "answerId": "L7izf6lVoyY9QyERd0m2",            "clerkId": 4625,            "headerimg": "/resource/2016/8/26/c160c426-a.jpeg",            "name": "林建麗",            "content": "合生元益生菌",            "createtime": NumberLong(1488302318220),            "adopt": 0,            "score": 0,            "zan": 0,            "ids": [                            ],            "clerkAnswerScore": [                            ],            "qid": "58e48e32830f398e8f401a95",            "status": 0,            "isReward": 0,            "replayAnswers": [                            ]        }    ]}

當要只查詢clerkAnswer這個數組中的某條資料時可以這樣查find({"clerkAnswer._id":"GkJtDMB7uizt699qFyEhAINZ0"},{"clerkAnswer.$":{$slice:1}}) ，如果你在<查詢條件>中查詢的內容是array裡的內容，用$就能引用前查詢中匹配到的元素。所以不需要遍曆就能直接取到數組中的某一個對象。如果把clerkAnswer.$改為clerkAnswer，查詢到的就是三條記錄中的第一條了，而不是前邊指定的那條,當然如果你前邊的查詢條件沒有具體包含到數組裡的資料比如你是這樣查的 find({"_id":ObjectId("58e48e32830f398e8f401a95")},{"clerkAnswer.$":{$slice:1}})這樣就查不到clerkAnswer中指定的某條了

mongodb "$"的作用