Use $size on all documents in array MongoDB

Question

Here's the structure part of my collection:

_id: ObjectId("W"),
names: [
    {
        number: 1,
        list: ["A","B","C"]
    },
    {
        number: 2,
        list: ["B"]
    },
    {
        number: 3,
        list: ["A","C"]
    }
    ...
],
...

I use this request:

db.publication.aggregate( [ { $match: { _id: ObjectId("54a1de90453d224e80f5fc60") } }, { $group: { _id: "$_id", SizeName: { $first: { $size: { $ifNull: [ "$names", [] ] } } }, names: { $first: "$names" } } } ] );

but I would now use $size in every documents of my names array.

Is it possible to get this result (where "sizeList" is the result of $size) :

_id: ObjectId("W"),
SizeName: 3,
names: [
    {
        SizeList: 3,
        number: 1,
        list: ["A","B","C"]
    },
    {
        SizeList: 1,
        number: 2,
        list: ["B"]
    },
    {
        SizeList: 2,
        number: 3,
        list: ["A","C"]
    }
    ...
],
...

Neil Lunn · Accepted Answer · 2015-01-11 23:51:10Z

4

All you really want here is a $project stage and use $map to alter the contents of the array members:

db.names.aggregate([
    { "$project": {
        "SizeName": { "$size": "$names" },
        "names": { "$map": {
            "input": "$names",
            "as": "el",
            "in": {
                "SizeList": { "$size": "$$el.list" },
                "number": "$$el.number",
                "list": "$$el.list"
            }
        }}
    }}
])

You can alternately process with $unwind and group it all back again, but that's kind of long winded when you have MongoDB 2.6 anyway.

answered Jan 11, 2015 at 23:51

Neil Lunn

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Mark Dickson Jr. · Accepted Answer · 2015-01-11 23:51:02Z

0

This isn't necessarily the most efficient way, but I think it does what you're aiming to do:

db.publication.aggregate( [ 
    { $unwind: '$names' },
    { $unwind: '$names.list' },
    { $group: {
        _id: { _id: "$_id", number: "$names.number" },
        SizeList: { $sum: 1 },
        list: { $push: "$names.list" }
      }
    },
    { $group: {
        _id: "$_id._id",
        names: {
            $push: {
                number: "$_id.number",
                list: "$list",
                SizeList: "$SizeList"
            }
        },
        SizeName: {$sum: 1}
      }
    }
]);

answered Jan 11, 2015 at 23:51

Mark Dickson Jr.

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2 Comments

Neil Lunn Over a year ago

You might not have been aware that the $size operator referred to by the OP in their question is a MongoDB 2.6 and greater feature that efficiently reports the size of an array. So it is no longer necessary to unwind arrays just to count the elements.

Mark Dickson Jr. Over a year ago

Yeah, your answer is awesome, Neil - Thanks

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