mongodb查询案例，golang库mgo案例

pipeline := []bson.M{   bson.M{"$match": bson.M{"id": uid}},   bson.M{"$project": bson.M{"last_msg": 1, "_id": 0}},   bson.M{"$unwind": "$last_msg"},   bson.M{"$sort": bson.M{"last_msg": -1}},   bson.M{"$skip": 2},   bson.M{"$limit": 2},}userModel := MongoDb.C("user")pipe := userModel.Pipe(pipeline)var data []interface{}err := pipe.All(&data)fmt.Println(data)fmt.Println(len(data))

等效于(查询id为1，只取出last_msg字段，不取_id，并将last_msg的数组取出展开，按last_msg.date逆序排列，跳过前2条，只选取2条)：

db.getCollection("user").aggregate([    {$match: {"id":"1"}},    {$project:{"last_msg":1, "_id":0}},    {$unwind: "$last_msg"},    {$sort: {"last_msg.date":-1}},    {$skip:2},    {$limit:2}]);

mongodb表结构：

其中一条数据：

{    "_id": ObjectId("5d2b24c16197934ef6db77ba"),    "id": "1",    "account": "你的女神",    "passwd": "5d78eb174d633345054faf7d56a612ed",    "friends": [        {            "uid": "2"        },        {            "uid": "3"        }    ],    "last_msg": [        {            "date": "1234567890",            "from_id": "2",            "msg": "在么"        },        {            "date": "1234567895",            "from_id": "3",            "msg": "女神"        },        {            "date": "1234567898",            "from_id": "4",            "msg": "求你了，理理我好么"        },        {            "date": "1234567999",            "from_id": "5",            "msg": "你是不是在和别人..."        }    ]}