考虑以下示例集合:

 {
    "_id:"0,
    "firstname":"Tom",
    "children" : {
                    "childA":{
                                "toys":{
                                        'toy 1':'batman',
                                        'toy 2':'car',
                                        'toy 3':'train',
                                        }
                                "movies": {
                                        'movie 1': "Ironman"
                                        'movie 2': "Deathwish"
                                        }
                                },
                    "childB":{
                                "toys":{
                                        'toy 1':'doll',
                                        'toy 2':'bike',
                                        'toy 3':'xbox',
                                        }
                                "movies": {
                                        'movie 1': "Frozen"
                                        'movie 2': "Barbie"
                                        }
                                }
                    }
}

movies = users.find_one({'_id': 0}, {'_id': 0, 'children.ChildA.movies': 1})

                                       {
                                        'movie 1': "Frozen"
                                        'movie 2': "Barbie"
                                        }

最佳答案

这里的问题是您当前的数据结构对于查询而言确实不是很好。这主要是因为您使用的是“键”来实际表示“数据点”，尽管最初看起来似乎是一个合理的想法，但这实际上是一个非常糟糕的做法。

因此，与其做诸如将“childA”和“childB”分配为对象或“子文档”的键之类的事情，不如将这些“值”分配给通用键名，例如:

 {
    "_id:"0,
    "firstname":"Tom",
    "children" : [
        { 
            "name": "childA", 
            "toys": [
                "batman",
                "car",
                "train"
            ],
            "movies": [
                "Ironman"
                "Deathwish"
            ]
        },
        {
            "name": "childB",
            "toys": [
                "doll",
                "bike",
                "xbox",
            ],
            "movies": [
                "Frozen",
                "Barbie"
            ]
        }
    ]
}

children -> childA -> toys
children -> childB -> toys

"children.toys"

db.collection.aggregate([
    # De-normalize the array content first
    { "$unwind": "$children" },

    # De-normalize the content from the inner array as well
    { "$unwind": "$children.movies" },

    # Group back, well optionally, but just the "movies" per document
    { "$group": {
        "_id": "$_id",
        "movies": { "$addToSet": "$children.movies" }
    }}
])

db.collection.find({},{ "_id": False, "children.movies": True })

db.collection.distinct("children.toys")

 db.collection.mapReduce(
     // Mapper
     function() {
         var id this._id;
             children = this.children;

         Object.keys(children).forEach(function(child) {
             Object.keys(child).forEach(function(childKey) {
                 Object.keys(childKey).forEach(function(toy) {
                     emit(
                         id, { "toys": [children[childkey]["toys"][toy]] }
                     );
                 });
             });
         });
     },
     // Reducer
     function(key,values) {
         var output = { "toys": [] };

         values.forEach(function(value) {
             value.toys.forEach(function(toy) {
                 if ( ouput.toys.indexOf( toy ) == -1 )
                     output.toys.push( toy );
             });
         });
     },
     {
         "out": { "inline": 1 }
     }
)

 {
    "_id:"0,
    "firstname":"Tom",
    "childtoys" : [
        { 
            "name": "childA", 
            "toy": "batman"
        }.
        { 
            "name": "childA",
            "toy": "car"
        },
        {
            "name": "childA",
            "toy": "train"
        },
        {
            "name": "childB",
            "toy": "doll"
        },
        {
            "name": "childB",
            "toy": "bike"
        },
        {
            "name": "childB",
            "toy": "xbox"
        }
    ],
    "childMovies": [
        {
             "name": "childA"
             "movie": "Ironman"
       },
       {
            "name": "childA",
            "movie": "Deathwish"
       },
       {
            "name": "childB",
            "movie": "Frozen"
       },
       {
            "name": "childB",
            "movie": "Barbie"
       }
  ]
}

关于javascript - MongoDb:如何从文档中获取字段(子文档)？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25909927/