Mongoose查询结果过滤异常：误删归属正确的instances对象

我通过Mongoose的find查询得到一个展览对象数组，结构如下（每个展览包含artworks数组，每个艺术品又包含instances数组）：

let exhibitions = [
    {
      "_id": "63047848edbb342f6805166e",
      "name":"Exhibition1",         
      "artworks": [
          {
            "_id": "6303519929e74916708b508b",             
            "instances": [
                {
                  "_id": "630486e2a13932fa8e2ff4b5",
                  "parentID": "6303519929e74916708b508b",
                  "exhibitionID": "63047848edbb342f6805166e",
                  "HangLineID": "555",
                },
                {
                  "_id": "630486e2a13932fa8e2ff4b5",
                  "parentID": "6303519929e74916708b508b",
                  "exhibitionID": "63047848edbb342f6805166e",
                  "HangLineID": "444",
                },
            ]
          },
          {
           "_id": "6304878ba13932fa8e2ff4d9",
            "instances": [
                {
                  "_id": "6308377d7c60564627494e16",
                  "parentID": "6304878ba13932fa8e2ff4d9",
                  "exhibitionID": "6304dbfb2af05390425969f7",
                  "HangLineID": "333",
                }
            ]           
          },               
      ],         
    },
    {           
      "_id": "6304dbfb2af05390425969f7", 
      "name":"Exhibition2",         
      "artworks": [
          {
            "_id": "62f20e22dc17b1712596016a",                  
            "instances": [
                {
                  "_id": "6304dc672af0539042596a07",
                  "parentID": "62f20e22dc17b1712596016a",
                  "exhibitionID": "6304dbfb2af05390425969f7",
                  "HangLineID": "000",
                  "HangLineArtworkArrayPosition": 2,
                  "__v": 0
                }
            ]
          }
      ]
    }
]

需求是：移除instances中exhibitionID与所属展览_id不匹配的元素（两者均为Mongoose ObjectId类型）。

尝试用Lodash的remove方法实现时，成功移除了Exhibition1中不符合的实例，但误删了Exhibition2中归属正确的实例。我的代码如下：

exhibitions.map((exh, key)=>{ 
  const artworks = exh.artworks;
  artworks.map((artwork,index1) =>{
    const instances = artwork.instances
    _.remove(
      instances, 
      v => (
        v.exhibitionID.toString()!=(exh._id).toString())            
      )})
      
    })

经测试，v.exhibitionID.toString()!==(exh._id).toString()的条件判断输出是正确的；改用v.HangLineID!="555"时能正常过滤，怀疑是对象比较逻辑的问题？

核心问题：闭包变量引用导致的循环异常 #

你用map遍历artworks时，内部的_.remove回调里引用了外部的exh变量，但由于JavaScript闭包的特性，当循环执行到后期时，回调里的exh可能已经指向数组的最后一个元素（也就是Exhibition2），导致所有instances都用最后一个展览的_id做判断，从而误删Exhibition2中原本符合条件的实例。

修复方案 #

可以通过以下两种方式解决：

方案1：用forEach替代map（避免闭包引用问题）

map的设计目的是返回新数组，这里我们只是要修改原数组，用forEach更合适，同时也能避免闭包变量的引用异常：

exhibitions.forEach(exh => {
  exh.artworks.forEach(artwork => {
    const currentExhId = exh._id.toString();
    _.remove(artwork.instances, v => v.exhibitionID.toString() !== currentExhId);
  });
});

方案2：提前缓存当前展览的ID，避免闭包引用

如果坚持用map，可以提前把当前展览的ID转成字符串缓存起来，确保回调里用的是当前循环的ID：

exhibitions.map(exh => { 
  const currentExhId = exh._id.toString();
  exh.artworks.map(artwork => {
    _.remove(artwork.instances, v => v.exhibitionID.toString() !== currentExhId);
  });
  return exh; // map需要返回元素，否则会生成全为undefined的数组
});

额外优化：用Mongoose内置方法比较ObjectId #

Mongoose的ObjectId本身提供了equals方法，可以直接用来比较，不需要转字符串，这样更可靠：

// 用equals方法替代toString比较
_.remove(artwork.instances, v => !v.exhibitionID.equals(exh._id));

内容的提问来源于stack exchange，提问作者Miss_scare_all