如何通过 MongoDb 中的嵌套对象字段加入集合？答案

【问题标题】：How to join collection by nested object field in MongoDb?如何通过 MongoDb 中的嵌套对象字段加入集合？
【发布时间】：2021-09-03 09:55:06
【问题描述】：

我有一个来自集合“用户”的示例项目：

{
    "_id" : ObjectId("612e8361942c250f4ca280f3"),
    "_cart" : [ 
        {
            "_id" : "60e38a2f2f6338e00e4ceff6",
            "count" : 2
        }, 
        {
            "_id" : "60e38a2f2f6338e00e4ceff7",
            "count" : 3
        }, 
        {
            "_id" : "60e38a2f2f6338e00e4ceff8",
            "count" : 4
        }
    ],
    "name" : "max1",
    "email" : "andrey.kupets1@i.ua"
}

我有这样的产品集合：

/* 1 */
{
    "_id" : ObjectId("60e38a2f2f6338e00e4ceff6"),
    "name" : "Куб-лабиринт Svoora Замок",
    "category" : "Настольные игры",
    "price" : 449.0
}

/* 2 */
{
    "_id" : ObjectId("60e38a2f2f6338e00e4ceff7"),
    "name" : "Настольная игра Hasbro Классическая Монополия обновленная (C1009_121)",
    "category" : "Настольные игры",
    "price" : 449.0
}

/* 3 */
{
    "_id" : ObjectId("60e38a2f2f6338e00e4ceff8"),
    "name" : "Настольный футбол Merchant Ambassador 2-в-1",
    "category" : "Настольные игры",
    "price" : 1899.0
}

如何通过在_cart-field 中嵌套_id-field 将产品对象加入到用户集合中的_cart-field？
我尝试以这种方式加入：

db.getCollection('users').aggregate([
    {$lookup: {
        from: 'products',
        localField: '_cart',
        foreignField: '_id',
        as: '_productsInCart'
    }}
])

我期待下一个输出：

{
    "_id" : ObjectId("612e890a3bb5da0328c4ffe4"),
    "_cart" : [ 
        {
            "_id" : "60e38a2f2f6338e00e4ceff6",
            "count" : 2
        }, 
        {
            "_id" : "60e38a2f2f6338e00e4ceff7",
            "count" : 3
        }, 
        {
            "_id" : "60e38a2f2f6338e00e4ceff8",
            "count" : 4
        }
    ],
    "name" : "max1",
    "email" : "andrey.kupets1@i.ua",
    "_productsInCart" : [ 
        {
            "_id" : ObjectId("60e38a2f2f6338e00e4ceff6"),
            "name" : "Куб-лабиринт Svoora Замок",
            "category" : "Настольные игры",
            "count" : 2,    
            "price" : 449.0,
        }, 
        {
            "_id" : ObjectId("60e38a2f2f6338e00e4ceff7"),
            "name" : "Настольная игра Hasbro Классическая Монополия обновленная (C1009_121)",
            "category" : "Настольные игры",
            "count" : 3,
            "price" : 449.0,
        }, 
        {
            "_id" : ObjectId("60e38a2f2f6338e00e4ceff8"),
            "name" : "Настольный футбол Merchant Ambassador 2-в-1",
            "category" : "Настольные игры",
            "count" : 4,
            "price" : 1899.0,
        }
    ]
}

但实际上我在 _productsInCart 字段中有输出：

"_productsInCart" : [ ]

【问题讨论】：

标签： mongodb join collections aggregate

【解决方案1】：

您需要使用以下聚合：请参考https://mongoplayground.net/p/FfWpFZKdxhV

db.users.aggregate([
  {
    "$unwind": "$_cart"
  },
  {
    "$addFields": {
      "_cartObjId": {
        "$toObjectId": "$_cart._id"
      }
    }
  },
  {
    $lookup: {
      from: "products",
      localField: "_cartObjId",
      foreignField: "_id",
      as: "_productsInCart"
    }
  },
  {
    "$unwind": "$_productsInCart"
  },
  {
    "$group": {
      "_id": "$_id",
      "_cart": {
        "$push": "$_cart"
      },
      "_productsInCart": {
        "$push": "$_productsInCart"
      },
      
    }
  }
])

【讨论】：

超级！谢谢！