使用$ slice运算符获取mongodb中数组的最后一个元素

Question

如何使用mongodb中的条件获取数组的最后一个元素.我无法使用切片.这是我的代码

{ "1" : { "relevancy" : [  "Y" ] }, "_id" : ObjectId("530824b95f44eac1068b45c0") }
{ "1" : { "relevancy" : [  "Y",  "Y" ] }, "_id" : ObjectId("530824b95f44eac1068b45c2") }
{ "1" : { "relevancy" : [  "N" ] }, "_id" : ObjectId("530824b95f44eac1068b45c3") }
{ "1" : { "relevancy" : [  "Y",  "Y" ] }, "_id" : ObjectId("530824b95f44eac1068b45c4") }
{ "1" : { "relevancy" : [  "Y",  "N" ] }, "_id" : ObjectId("530824b95f44eac1068b45c6") }
{ "1" : { "relevancy" : [  "N" ] }, "_id" : ObjectId("530824b95f44eac1068b45c7") }
{ "1" : { "relevancy" : [  "Y",  "N" ] }, "_id" : ObjectId("530824b95f44eac1068b45c8") }

我想计算具有"Y"的行数作为数组"相关性"的最后一个元素.从上面的代码应该是3.如何做到这一点.请帮助我.

Answer 1

正如您现在所知,$ slice仅用于投影以限制结果中返回的数组元素.因此,您将无法使用find()的结果以编程方式处理列表.

更好的方法是使用聚合.但首先让我们考虑如何使用$ slice:

> db.collection.find({},{ relevancy: {$slice: -1} })
{ "_id" : ObjectId("530824b95f44eac1068b45c0"), "relevancy" : [  "Y" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c2"), "relevancy" : [  "Y" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c3"), "relevancy" : [  "N" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c4"), "relevancy" : [  "Y" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c6"), "relevancy" : [  "N" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c7"), "relevancy" : [  "N" ] }
{ "_id" : ObjectId("530824b95f44eac1068b45c8"), "relevancy" : [  "N" ] }

因此,您获得了最后一个数组元素,但由于无法匹配最后一个元素值,因此您无法循环结果.您也可以在代码中完成此操作.

现在让我们来看看聚合:

db.collection.aggregate([
    // Match things so we get rid of the documents that will never match, but it will
    // still keep some of course since they are arrays, that *may* contain "N"
    { "$match": { "relevancy": "Y" } },

    // De-normalizes the array
    { "$unwind": "$relevancy" },

    // The order of the array is retained, so just look for the $last by _id
    { "$group": { "_id": "$_id", "relevancy": { "$last": "$relevancy" } }},

    // Match only the records with the results you want
    { "$match": { "relevancy": "Y" }},

    // Oh, and maintain the original _id order [ funny thing about $last ]
    { "$sort": { "_id": 1 } }
])

即使这是你第一次使用aggregate(),我也鼓励你去学习它.它可能是您最有用的问题解决工具.当然一直适合我.如果您正在学习,请每次执行一次.

在您的文档表格中也不确定,所有1: { ... }子文档符号似乎都是错误的,但您应该清除或调整上面的代码以引用"1.relevancy".我希望你的文件看起来更像这样:

{ "relevancy" : [  "Y" ] , "_id" : ObjectId("530824b95f44eac1068b45c0") }
{ "relevancy" : [  "Y",  "Y" ] , "_id" : ObjectId("530824b95f44eac1068b45c2") }
{ "relevancy" : [  "N" ], "_id" : ObjectId("530824b95f44eac1068b45c3") }
{ "relevancy" : [  "Y",  "Y" ], "_id" : ObjectId("530824b95f44eac1068b45c4") }
{ "relevancy" : [  "Y",  "N" ], "_id" : ObjectId("530824b95f44eac1068b45c6") }
{ "relevancy" : [  "N" ], "_id" : ObjectId("530824b95f44eac1068b45c7") }
{ "relevancy" : [  "Y",  "N" ], "_id" : ObjectId("530824b95f44eac1068b45c8") }

MongoDB 3.2.x及更高版本

当然,MongoDB 3.2引入了一个"聚合"运算符,$slice并且是一个更好的$arrayElemAt运算符,无需任何处理$unwind和$group处理.在初始$match查询之后,您只需进行"逻辑匹配" $redact:

db.collection.aggregate([
    { "$match": { "relevancy": "Y" } },
    { "$redact": {
        "$cond": {
            "if": { "$eq": [{ "$arrayElemAt": [ "$relevancy", -1 ], "Y" ] },
            "then": "$$KEEP",
            "else": "$$PRUNE"
        }
    }}   
])

这将在决定是否$$KEEP或$$PRUNE返回结果中的文档时对数组的最后一个元素进行检查.

如果您仍然想要"投影",那么您实际上可以添加$slice:

db.collection.aggregate([
    { "$match": { "relevancy": "Y" } },
    { "$redact": {
        "$cond": {
            "if": { "$eq": [{ "$arrayElemAt": [ "$relevancy", -1 ], "Y" ] },
            "then": "$$KEEP",
            "else": "$$PRUNE"
        }
    }},
    { "$project": { "relevancy": { "$slice": [ "$relevancy", -1 ] } } }
])

或者替代方法:

db.collection.aggregate([
    { "$match": { "relevancy": "Y" } },
    { "$project": { "relevancy": { "$slice": [ "$relevancy", -1 ] } } },
    { "$match": { "relevancy": "Y" } }
])

但是$redact第一次做的成本可能更低,"然后"在$ $项目中进行任何重新塑造.