MongoDB：选择与子集合相匹配的元素

Question

我正在使用mongoose.js对mongodb执行查询，但我认为我的问题不是特定于mongoose.js。

说我有收集只有一条记录：

var album = new Album({ 
    tracks: [{ 
     title: 'track0', 
     language: 'en', 

    },{ 
     title: 'track1', 
     language: 'en', 
    },{ 
     title: 'track2', 
     language: 'es', 
    }] 
}) 

我想选择与语言字段等于“恩”所有的曲目，所以我尝试两种型号：

Album.find({'tracks.language':'en'}, {'tracks.$': 1}, function(err, albums){ 

和绑定到与elemMatch投影相同的东西：

Album.find({}, {tracks: {$elemMatch: {'language': 'en'}}}, function(err, albums){ 

在这两种情况下我得到了同样的结果：

{tracks:[{title: 'track0', language: 'en'}]} 

选择album.tracks包含标题为 'track0' 只有一个线路元件（但应该有两个 'track0'， 'TRACK1'）：

{tracks:[{title: 'track0', language: 'en'}, {title: 'track1', language: 'en'}]} 

我在做什么错？

Answer 1

就像@JohnnyHK已经说过的那样，您必须使用聚合框架才能实现该目标，因为$和$elemMatch只返回第一个匹配项。

方法如下：

db.Album.aggregate(
    // This is optional. It might make your query faster if you have 
    // many albums that don't have any English tracks. Take a larger 
    // collection and measure the difference. YMMV. 
    { $match: {tracks: {$elemMatch: {'language': 'en'}} } }, 

    // This will create an 'intermediate' document for each track 
    { $unwind : "$tracks" }, 

    // Now filter out the documents that don't contain an English track 
    // Note: at this point, documents' 'tracks' element is not an array 
    { $match: { "tracks.language" : "en" } }, 

    // Re-group so the output documents have the same structure, ie. 
    // make tracks a subdocument/array again 
    { $group : { _id : "$_id", tracks : { $addToSet : "$tracks" } }} 
); 

你可能想尝试，聚集查询，只有第一个表达式，然后通过行添加表情网上看到的输出如何变化。了解$unwind如何创建后期使用$group和$addToSet重新合并的中间文档特别重要。

结果：

> db.Album.aggregate(
    { $match: {tracks: {$elemMatch: {'language': 'en'}} } }, 
    { $unwind : "$tracks" }, 
    { $match: { "tracks.language" : "en" } }, 
    { $group : { _id : "$_id", tracks : { $addToSet : "$tracks" } }} ); 
{ 
    "result" : [ 
      { 
        "_id" : ObjectId("514217b1c99766f4d210c20b"), 
        "tracks" : [ 
          { 
            "title" : "track1", 
            "language" : "en" 
          }, 
          { 
            "title" : "track0", 
            "language" : "en" 
          } 
        ] 
      } 
    ], 
    "ok" : 1 
}