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"
            }
        ]
    }
]

 db.getCollection('aj').find({"shapes.color":"red"},{"shapes.$":1})

输出

{

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

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']]}
     }}
  }}
])

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"
    }
]}

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")
        }
    ]
}

你只需要运行query

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

此查询的输出为

{
    "_id" : ObjectId("562e7c594c12942f08fe4192"),
    "shapes" : [ 
        {"shape" : "circle", "color" : "red"}
    ]
}

正如你所期望的那样，它会从数组中给出匹配颜色的精确字段:'red'。