MongoDB:聚合$项目采取数组当前项目
嗨,我有这样的MongoDB查询。
.aggregate( [ { $match : { comment_box : "somdata" } }, { $project : { comment : 1, created_at: 1, current_date: "$replies.created_at", }, dateDifference: { $subtract: [ new Date(), "$created_at"] } }}, ] )
想象一下,我想从当前date字段的回复数组中获取created_at值。 但是这个查询返回
current_date: [ "2016-03-08T13:48:27.882Z", "2016-03-08T14:26:22.194Z" ]
而不是此数组中每个元素的当前created_at值
我尝试了很多方法,但有错误
current_date: "$replies.0.created_at", current_date: "$replies.$.created_at", current_date: "$$replies.created_at",
等等
请帮我检索这样的数据
current_date:"2016-03-08T13:48:27.882Z",
我假设你想要最新的评论date。 在这种情况下,你可以把$max
:
current_date: { $max: '$replies.created_at' }
演示:
> db.comments.insert({_id: 0, replies: [{created_at: new Date('2017-04-03')}, {created_at: new Date('2017-02-03')} ]}) WriteResult({ "nInserted" : 1 }) > db.comments.insert({_id: 1, replies: []}) WriteResult({ "nInserted" : 1 }) > db.comments.aggregate([{$project: {current_date: {$max: '$replies.created_at'}}}]) { "_id" : 0, "current_date" : ISODate("2017-04-03T00:00:00Z") } { "_id" : 1, "current_date" : null }
注意具有空replies
数组的文档如何获得null
。