$unwind：將文件中的某一個數組型別欄位拆分成多條，每條包含陣列中的一個值。
需求：

{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : [ 
        1, 
        2, 
        3, 
        4, 
        5
    ]
}

對weekday進行拆分:

db.getCollection('chenxiaochantest').aggregate(
 [
 

  {
     $unwind:"$weekday"
  }
 ]
)

拆分結果:

/* 1 */
{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : 1
}
 
/* 2 */
{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : 2
}
 
/* 3 */
 

{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : 3
}
 
/* 4 */
{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : 4
}
 
/* 5 */
{
    "_id" : ObjectId("5951c5de567ebff0d5011fba"),
    "name"
 
 : "陳曉嬋",
    "address" : "北京朝陽區",
    "weekday" : 5
}

使用$unwind可以將weekday中的每個資料都被分解成一個文件,並且除了weekday的值不同外,其他的值都是相同的.

例項講解:

{
    "_id" : ObjectId("5951ca15567ebff0d5011fbb"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽",
    "lunch" : [ 
        {
            "food" : "baozi",
            "fruit" : "taozi"
        }, 
        {
            "food" : "miaotiao",
            "fruit" : "xigua"
        }
    ]
}

對lunch進行拆分:

db.getCollection('chenxiaochantest2').aggregate(
  [
    {
       $unwind:"$lunch"
    }
  ]
)

拆封結果:

/* 1 */
{
    "_id" : ObjectId("5951ca15567ebff0d5011fbb"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽",
    "lunch" : {
        "food" : "baozi",
        "fruit" : "taozi"
    }
}
 
/* 2 */
{
    "_id" : ObjectId("5951ca15567ebff0d5011fbb"),
    "name" : "陳曉嬋",
    "address" : "北京朝陽",
    "lunch" : {
        "food" : "miaotiao",
        "fruit" : "xigua"
    }
}

轉載來源：https://blog.csdn.net/chenxiaochan/article/details/73771879