使用聚合函数和$project获取文档中的特定对象字段

db.getCollection('geolocations').aggregate([ { $project : { geolocation : 1} } ])

结果:

{
    "_id" : ObjectId("5e3ee15968879c0d5942464b"),
    "geolocation" : [ 
        {
            "_id" : ObjectId("5e3ee3ee68879c0d5942465e"),
            "latitude" : 12.9718313,
            "longitude" : 77.593551,
            "country" : "India",
            "city" : "Chennai",
            "zipcode" : "560001",
            "streetName" : "Sidney Road",
            "countryCode" : "in",
            "ip" : "116.75.115.248",
            "date" : ISODate("2020-02-08T16:38:06.584Z")
        }
    ]
}

mongodb中的find语法是

    db.<collection name>.find(query, projection);

你写的第二个查询

    db.test.find(
    {shapes: {"$elemMatch": {color: "red"}}}, 
    {"shapes.color":1})

在这里，你已经在查询部分使用了$elemMatch操作符，而如果你在投影部分使用这个操作符，那么你将得到想要的结果。您可以将您的查询写成

     db.users.find(
     {"shapes.color":"red"},
     {_id:0, shapes: {$elemMatch : {color: "red"}}})

这会给你想要的结果。

同样，你也可以求出倍数

db.getCollection('localData').aggregate([
    // Get just the docs that contain a shapes element where color is 'red'
  {$match: {'shapes.color': {$in : ['red','yellow'] } }},
  {$project: {
     shapes: {$filter: {
        input: '$shapes',
        as: 'shape',
        cond: {$in: ['$$shape.color', ['red', 'yellow']]}
     }}
  }}
])

MongoDB 2.2新的$elemMatch投影操作符提供了另一种方法来修改返回的文档，使其只包含第一个匹配的形状元素:

db.test.find(
    {"shapes.color": "red"}, 
    {_id: 0, shapes: {$elemMatch: {color: "red"}}});

返回:

{"shapes" : [{"shape": "circle", "color": "red"}]}

在2.2中，还可以使用$ projection操作符，其中投影对象字段名中的$表示查询中该字段的第一个匹配数组元素的索引。下面返回与上面相同的结果:

db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1});

MongoDB 3.2更新

从3.2版本开始，您可以使用新的$filter聚合操作符在投影期间筛选数组，它的好处是包括所有匹配，而不仅仅是第一个匹配。

db.test.aggregate([
    // Get just the docs that contain a shapes element where color is 'red'
    {$match: {'shapes.color': 'red'}},
    {$project: {
        shapes: {$filter: {
            input: '$shapes',
            as: 'shape',
            cond: {$eq: ['$$shape.color', 'red']}
        }},
        _id: 0
    }}
])

结果:

[ 
    {
        "shapes" : [ 
            {
                "shape" : "circle",
                "color" : "red"
            }
        ]
    }
]

更多细节请参考=

Mongo db官方参考

suppose you have document like this (you can have multiple document too) - { "_id": { "$oid": "63b5cfbfbcc3196a2a23c44b" }, "results": [ { "yearOfRelease": "2022", "imagePath": "https://upload.wikimedia.org/wikipedia/en/d/d4/The_Kashmir_Files_poster.jpg", "title": "The Kashmir Files", "overview": "Krishna endeavours to uncover the reason behind his parents' brutal killings in Kashmir. He is shocked to uncover a web of lies and conspiracies in connection with the massive genocide.", "originalLanguage": "hi", "imdbRating": "8.3", "isbookMark": null, "originCountry": "india", "productionHouse": [ "Zee Studios" ], "_id": { "$oid": "63b5cfbfbcc3196a2a23c44c" } }, { "yearOfRelease": "2022", "imagePath": "https://upload.wikimedia.org/wikipedia/en/a/a9/Black_Adam_%28film%29_poster.jpg", "title": "Black Adam", "overview": "In ancient Kahndaq, Teth Adam was bestowed the almighty powers of the gods. After using these powers for vengeance, he was imprisoned, becoming Black Adam. Nearly 5,000 years have passed, and Black Adam has gone from man to myth to legend. Now free, his unique form of justice, born out of rage, is challenged by modern-day heroes who form the Justice Society: Hawkman, Dr. Fate, Atom Smasher and Cyclone", "originalLanguage": "en", "imdbRating": "8.3", "isbookMark": null, "originCountry": "United States of America", "productionHouse": [ "DC Comics" ], "_id": { "$oid": "63b5cfbfbcc3196a2a23c44d" } }, { "yearOfRelease": "2022", "imagePath": "https://upload.wikimedia.org/wikipedia/en/0/09/The_Sea_Beast_film_poster.png", "title": "The Sea Beast", "overview": "A young girl stows away on the ship of a legendary sea monster hunter, turning his life upside down as they venture into uncharted waters.", "originalLanguage": "en", "imdbRating": "7.1", "isbookMark": null, "originCountry": "United States Canada", "productionHouse": [ "Netflix Animation" ], "_id": { "$oid": "63b5cfbfbcc3196a2a23c44e" } }, { "yearOfRelease": "2021", "imagePath": "https://upload.wikimedia.org/wikipedia/en/7/7d/Hum_Do_Hamare_Do_poster.jpg", "title": "Hum Do Hamare Do", "overview": "Dhruv, who grew up an orphan, is in love with a woman who wishes to marry someone with a family. In order to fulfil his lover's wish, he hires two older individuals to pose as his parents.", "originalLanguage": "hi", "imdbRating": "6.0", "isbookMark": null, "originCountry": "india", "productionHouse": [ "Maddock Films" ], "_id": { "$oid": "63b5cfbfbcc3196a2a23c44f" } }, { "yearOfRelease": "2021", "imagePath": "https://upload.wikimedia.org/wikipedia/en/7/74/Shang-Chi_and_the_Legend_of_the_Ten_Rings_poster.jpeg", "title": "Shang-Chi and the Legend of the Ten Rings", "overview": "Shang-Chi, a martial artist, lives a quiet life after he leaves his father and the shadowy Ten Rings organisation behind. Years later, he is forced to confront his past when the Ten Rings attack him.", "originalLanguage": "en", "imdbRating": "7.4", "isbookMark": null, "originCountry": "United States of America", "productionHouse": [ "Marvel Entertainment" ], "_id": { "$oid": "63b5cfbfbcc3196a2a23c450" } } ], "__v": 0 } ======= mongo db query by aggregate command - mongomodels.movieMainPageSchema.aggregate( [ { $project: { _id:0, // to supress id results: { $filter: { input: "$results", as: "result", cond: { $eq: [ "$$result.yearOfRelease", "2022" ] } } } } } ] )

感谢JohnnyHK。

这里我只想添加一些更复杂的用法。

// Document 
{ 
"_id" : 1
"shapes" : [
  {"shape" : "square",  "color" : "red"},
  {"shape" : "circle",  "color" : "green"}
  ] 
} 

{ 
"_id" : 2
"shapes" : [
  {"shape" : "square",  "color" : "red"},
  {"shape" : "circle",  "color" : "green"}
  ] 
} 


// The Query   
db.contents.find({
    "_id" : ObjectId(1),
    "shapes.color":"red"
},{
    "_id": 0,
    "shapes" :{
       "$elemMatch":{
           "color" : "red"
       } 
    }
}) 


//And the Result

{"shapes":[
    {
       "shape" : "square",
       "color" : "red"
    }
]}