在MongoDB聚合中合并数组字段

本文介绍了在MongoDB聚合中合并数组字段的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

使用MongoDB聚合框架时是否可以合并数组字段?这是我要解决的摘要问题:

Is it possible to merge array fields in while using MongoDB aggregation framework? Here is a summary problem I am trying to solve:

抽样汇总输入文档:

{ "Category" : 1, "Messages" : ["Msg1", "Msg2"], "Value" : 1 }, { "Category" : 1, "Messages" : [], "Value" : 10 }, { "Category" : 1, "Messages" : ["Msg1", "Msg3"], "Value" : 100 }, { "Category" : 2, "Messages" : ["Msg4"], "Value" : 1000 }, { "Category" : 2, "Messages" : ["Msg5"], "Value" : 10000 }, { "Category" : 3, "Messages" : [], "Value" : 100000 }

我们想按类别"分组，同时总结价值"并合并消息".我已经尝试过以下聚合管道:

We want to group by 'Category' while summing up 'Value' and merging 'Messages'. I have tried this aggregation pipeline:

{group : { _id : "$Category", Value : { $sum : "$Value"}, Messages : {$push : "$Messages"} } }, {$unwind : "$Messages"}, {$unwind : "$Messages"}, {$group : { _id : "$_id", Value : {$first : "$Value"}, Messages : {$addToSet : "$Messages"} } }

结果是:

"result" : [{ "_id" : 1, "Value" : 111, "Messages" : ["Msg3", "Msg2", "Msg1"] }, { "_id" : 2, "Value" : 11000, "Messages" : ["Msg5", "Msg4"] } ]

但是，这完全错过了类别3，因为类别"为3的文档没有任何消息"，并且在第二次展开时将其删除.我们希望结果也包括以下内容:

However, this completely misses Category 3 since the documents where 'Category' is 3 do not have any 'Messages' and they are dropped by the second unwind. We would like the result to include the following as well:

{ "_id" : 3, "Value" : 100000, "Messages" : [] }

是否有通过聚合框架实现此目标的灵巧方法?

Is there a neat way of achieving this by the aggregation framework?

推荐答案

如果要确保Messages是数组，则可以使用以下技巧:

Here is a trick you can use if Messages is guaranteed to be an array:

> db.messages.find() { "Category" : 1, "Messages" : [ "Msg1", "Msg2" ], "Value" : 1 } { "Category" : 1, "Messages" : [ ], "Value" : 10 } { "Category" : 1, "Messages" : [ "Msg1", "Msg3" ], "Value" : 100 } { "Category" : 2, "Messages" : [ "Msg4" ], "Value" : 1000 } { "Category" : 2, "Messages" : [ "Msg5" ], "Value" : 10000 } { "Category" : 3, "Messages" : [ ], "Value" : 100000 } > var group1 = { "$group": { "_id": "$Category", "Value": { "$sum": "$Value" }, "Messages": { "$push": "$Messages" } } }; > var project1 = { "$project": { "Value": 1, "Messages": { "$cond": [ { "$eq": [ "$Messages", [ [ ] ] ] }, [ [ null ] ], "$Messages" ] } } }; > db.messages.aggregate( group1, project1 ) { "_id" : 3, "Value" : 100000, "Messages" : [ [ null ] ] } { "_id" : 2, "Value" : 11000, "Messages" : [ [ "Msg4" ], [ "Msg5" ] ] } { "_id" : 1, "Value" : 111, "Messages" : [ [ "Msg1", "Msg2" ], [ ], [ "Msg1", "Msg3" ] ] }

现在放开两次，然后重新分组以获得单个Messages数组.

Now unwind twice and re-group to get a single Messages array.

> var unwind = {"$unwind":"$Messages"}; > var group2 = { $group: { "_id": "$_id", "Value": { "$first": "$Value" }, "Messages": { "$addToSet": "$Messages" } } }; > var project2 = { "$project": { "Category": "$_id", "_id": 0, "Value": 1, "Messages": { "$cond": [ { "$eq": [ "$Messages", [ null ] ] }, [ ], "$Messages" ] } } }; > db.messages.aggregate(group1, project1, unwind, unwind, group2 ,project2 ) { "Value" : 111, "Messages" : [ "Msg3", "Msg2", "Msg1" ], "Category" : 1 } { "Value" : 11000, "Messages" : [ "Msg5", "Msg4" ], "Category" : 2 } { "Value" : 100000, "Messages" : [ ], "Category" : 3 }