MongoDB数组对象更新：存在则增量，不存在则新增

示例文档 #

{
    "_id" : "1",
    "principal" : "Joe",
    "classroom" : [
        {
            "grade" : "1",
            "class" : "A",
            "totalStudent" : 10.0
        },
        {
            "grade" : "1",
            "class" : "B",
            "totalStudent" : 20.0
        }
    ]
}

需求 #

• 当筛选条件匹配时，递增totalStudent字段，例如筛选条件为principal : "Joe", classroom.grade: "1", classroom.class: "A"
• 当目标对象不存在时，向classroom数组中新增对象，例如新增classroom.grade: "1", classroom.class: "C"对应的对象

现有查询（仅支持存在对象的增量操作） #

db.getCollection("classsroom").updateOne(
    {
        pricipal: "Joe", 
        "classroom": {$elemMatch: {"grade":"1","class": "A"}}        
    },
    
    { $inc: { "classroom.$.totalStudent": 5 } },

    {upsert: true}
)

解决方案 #

要同时实现匹配元素递增、不匹配则新增的需求，使用聚合管道更新语法即可一步完成：

db.getCollection("classroom").updateOne(
    { principal: "Joe" },
    [
        {
            $set: {
                classroom: {
                    $cond: {
                        if: {
                            $ne: [
                                { $filter: { input: "$classroom", cond: { $and: [{ $eq: ["$$this.grade", "1"] }, { $eq: ["$$this.class", "C"] }] } } },
                                []
                            ]
                        },
                        then: {
                            $map: {
                                input: "$classroom",
                                in: {
                                    $cond: {
                                        if: { $and: [{ $eq: ["$$this.grade", "1"] }, { $eq: ["$$this.class", "C"] }] },
                                        then: { $mergeObjects: ["$$this", { totalStudent: { $add: ["$$this.totalStudent", 5] } }] },
                                        else: "$$this"
                                    }
                                }
                            }
                        },
                        else: {
                            $concatArrays: [
                                "$classroom",
                                [{ grade: "1", class: "C", totalStudent: 5 }]
                            ]
                        }
                    }
                }
            }
        }
    ],
    { upsert: true }
)

逻辑说明 #

1. 外层条件{ principal: "Joe" }定位目标文档
2. 通过$filter判断数组中是否存在指定grade和class的元素
3. 若存在，用$map遍历数组，对匹配元素执行totalStudent递增操作，其他元素保持原样
4. 若不存在，用$concatArrays向数组末尾新增包含初始值的目标对象

期望执行结果 #

{
    "_id" : "1",
    "principal" : "Joe",
    "classroom" : [
        {
            "grade" : "1",
            "class" : "A",
            "totalStudent" : 10.0
        },
        {
            "grade" : "1",
            "class" : "B",
            "totalStudent" : 20.0
        },
        {
            "grade" : "1",
            "class" : "C",
            "totalStudent" : 5.0
        }
    ]
}

内容的提问来源于stack exchange，提问作者Rungkrit Titachotnimit