Mongo DB query: project array element to property

Question

Having following data

{
   "_id": "...",
   "name": "John",
   "purchases": [
       {
            "date": {
                "$date": "2016-12-20T00:00:00.000Z"
            },
            "amount": 1000
       },
       {
            "date": {
                "$date": "2016-12-23T00:00:00.000Z"
            },
            "amount": 100
       }    

   ]
}

How to get latest purchase amount as result field with different name?

To get next result:

{
    "_id": "...",
    "name": "John",
    "purchases": [
           {
                "date": {
                    "$date": "2016-12-20T00:00:00.000Z"
                },
                "amount": 1000
           },
           {
                "date": {
                    "$date": "2016-12-23T00:00:00.000Z"
                },
                "amount": 100
           }    

       ]
    },
    "latestPurchaseAmount": 100
}

G. Mansour · Accepted Answer · 2016-10-21 14:14:50Z

1

You have to use the $rename take a look at this link

And for your last item you should look at this it will helps you Link

edited Oct 21, 2016 at 14:14

answered Oct 21, 2016 at 14:07

G. Mansour

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1 Comment

STO Over a year ago

$rename seems to be an update operator. But I need to rename field due projection.

s7vr · Accepted Answer · 2016-10-24 13:13:45Z

1

You can do something like this. This below query will unwind all the purchases in the collection and, sort the purchases by date descending and, limit the result to one and project the corresponding amount.

db.purchases.aggregate([{
    $unwind: "$purchases"
}, {
    $sort: {
        "purchases.date": -1
    }
}, {
    $limit: 1
}, {
    $project: {
        _id: 0,
        latestPurchaseAmount: "$purchases.amount"
    }
}]);

Output

{ "latestPurchaseAmount" : 100 }

edited Oct 24, 2016 at 13:13

answered Oct 23, 2016 at 3:25

s7vr

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2 Comments

STO Over a year ago

Thanks, this looks like what I need. What about persons without purchases, will it be output? Is there a way to filter out amount without miss person?

s7vr Over a year ago

The persons without purchases will not be considered as $unwind won't return anything if the purchases array is empty or doesnt exist.

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