mongodb - 仅投影子文档中数组项的部分字段

Question

如何在子文档中仅投影数组中项目的特定字段？

考虑以下(简化的)示例:

{
    "_id" : ObjectId("573d70df080cc2cbe8bf3222"),
    "name" : "Nissan",
    "models" : [
        {
            "name" : "Altima",
            "body" : {
                 "type" : 2,
                 "maxprice" : 31800.00,
                 "minprice" : 21500.00
            }
        },
        {
             "name" : "Maxima",
             "body" : {
                 "type" : 2,
                 "maxprice" : 39200.00,
                 "minprice" : 28800.00
             }
        }
    ]
},
{
    "_id" : ObjectId("80cc2cbe8bf3222573d70df0"),
    "name" : "Honda",
    "models" : [
        {
            "name" : "Accord",
            "body" : {
                "type" : 2,
                "maxprice" : 34100.00,
                "minprice" : 20400.00
            }
        },
        {
            "name" : "Civic",
            "body" : {
                "type" : 3,
                "maxprice" : 27900.00,
                "minprice" : 19800.00
             }
        }
    ]
}

聚合后，我想得到如下输出:

{
    "_id" : ObjectId("573d70df080cc2cbe8bf3222"),
    "name" : "Nissan",
    "models" : [
        {
            "type" : 2,
            "minprice" : 21500.00
        },
        {
            "type" : 2,
            "minprice" : 28800.00
        }
    ]
},
{
    "_id" : ObjectId("80cc2cbe8bf3222573d70df0"),
    "name" : "Honda",
    "models" : [
        {
            "type" : 2,
            "minprice" : 20400.00
        },
        {
            "type" : 3,
            "minprice" : 19800.00
        }
    ]
}

所以它基本上获取了所有文档、文档的所有字段、models 数组中的所有项，但只是 models 中数组项的某些字段。请帮忙。

Answer 1

您需要$project “模型”字段使用 $map运营商。

db.collection.aggregate([ 
    { "$project": { 
        "name": 1, 
        "models": { 
            "$map": { 
                "input": "$models", 
                "as": "m", 
                "in": { 
                    "type": "$$m.body.type", 
                    "minprice": "$$m.body.minprice" 
                } 
            } 
        }  
    }} 
])