MongoDB aggregation : $sum values of a key in a collection

Question

I have a mongodb collection full of documents like this :

{
    _id : xxxxxx
    category : 1,
    tech : [
        {key:"size",value:5},
        {key:"color",value:"red"}
        {key:"weight",value:27.4}
    ]
}

My question is : how can I do to aggregate (average, sum or whatever) each item with key = "size" in this collection?

thank you for your help

Neil Lunn · Accepted Answer · 2014-04-09 14:23:34Z

2

When you have documents that contain an array you use the $unwind operator in order to access the array elements.

db.tech.aggregate([
    { "$unwind": "$tech" },
    { "$match": { "tech.key": "size" } },
    { "$group": { 
        "_id": null,
        "totalSize": { "$sum": "$tech.value" }
    }}
])

So once the array is "un-wound" you can then $group on whatever you want to use as a key under the _id field, for all documents in the collection use null. Any of the group aggregation operators can be applied.

The array elements in the "de-normalized" documents will be available through "dot notation" as shown above.

Also see the full list of aggregation operators in the manual.

edited Apr 9, 2014 at 14:23

answered Apr 4, 2014 at 22:57

Neil Lunn

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5 Comments

Fred Mériot Over a year ago

I tried your solution but it did not worked. I think you did not notice that my tech subdocument is an array of objects, so tech.size or tech.color is not accessible directly.

Neil Lunn Over a year ago

@FredMériot Don't know how that was missed in typing. See the edit. You need to filter in this particular case. There is another way to do this with the $cond operator as well

Neil Lunn Over a year ago

@FredMériot did you miss the edit comment. What about this does not meet your needs?

Fred Mériot Over a year ago

No, I did not succeed to make it works. I still have the problem with the $tech.value in the group directive.

Neil Lunn Over a year ago

@FredMériot possibly more typing mistakes but I do not see the problem in general. The statement works perfectly fine for me. Check the edit again.

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