MongoDB : aggregate $project take array current item

Question

Hi I have mongodb query like this .

.aggregate(
            [
                { $match : { comment_box : "somdata" } },

                { $project : {
                    comment : 1,                      
                        created_at: 1,
                        current_date: "$replies.created_at",
                    },
                    dateDifference: { $subtract: [ new Date(), "$created_at"] }
                }},
            ]
        )

Imagine that I want to get created_at value from replies array in current date field. but this query returns

current_date: [
   "2016-03-08T13:48:27.882Z",
   "2016-03-08T14:26:22.194Z"
]

instead of current created_at value of each element from this array

I have tried many ways but got errors

current_date: "$replies.0.created_at",
current_date: "$replies.$.created_at",
current_date: "$$replies.created_at",

and etc

please help me to retrieve data like this

current_date:"2016-03-08T13:48:27.882Z",

How do you determine the current date from an array of dates, is it always the first element in the array or you need to order the array descending then get the first item? — chridam
– chridam, Commented Mar 8, 2016 at 14:48

binki · Accepted Answer · 2017-07-09 23:55:39Z

1

I’m assuming you want the latest comment date. In that case, you can just take the $max:

current_date: { $max: '$replies.created_at' }

Demo:

> 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 }

Notice how the document with an empty replies array gets null.

answered Jul 9, 2017 at 23:55

binki

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