The role of MongoDB "$"

{    "_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": "Qi and Blood Two empty",    "Besttotal": 0,    "Bestscore": 0,    "Clerkanswer": [        {            "_id": "p0k94bz4ehv9xsftlmlozhs8l",            "Clerkid": 2159,            "Headerimg": "Head/2159/201612/1480922352820.jpeg",            "Name": "Medicine point shop length 7",            "Content": "High difficulty",            "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": "Medicine point Clerk 13",            "Content": "but",            "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": "Lin Jianli",            "Content": "co-sheng Yuan Probiotics",            "Createtime": Numberlong (1488302318220),            "Adopt": 0,            "Score": 0,            "Zan": 0,            "IDs": [                            ],            "Clerkanswerscore": [                            ],            "QID": "58e48e32830f398e8f401a95",            "Status": 0,            "Isreward": 0,            "Replayanswers": [                            ]        }    ]}

When you want to query only one piece of data in the Clerkanswer array, you can look for find ({"clerkanswer._id": "Gkjtdmb7uizt699qfyehainz0"},{"clerkanswer.$": {$slice : 1}), if you are querying the contents of the array in < query criteria >, you can refer to the elements that were matched in the previous query using $. So you can take an object directly into the array without traversing it. If you change the clerkanswer.$ to Clerkanswer, the query is the first of the three records, not the one specified in the front, of course, if the query conditions in front of you do not specifically include the data in the array, such as you find ({"_id": O Bjectid ("58e48e32830f398e8f401a95")},{"clerkanswer.$": {$slice: 1}}) This will not find the one specified in the Clerkanswer.

The role of MongoDB "$"