MongoDB count documents for each array elements

Question

I h

{
  code : "X1",
  elements : ["A", "B", "C", "D"]
},
{
  code : "X2",
  elements : ["C", "D"]
},
{
  code : "X3",
  elements : ["A"]
}
...

I would like to know the number of documents present for each type of value in the "elements" array. es. es.

"A" : 2
"B" : 1
"C" : 2
"D" : 2

is it possible with a single query?

mickl · Accepted Answer · 2019-12-15 23:04:02Z

4

You can $unwind your array to get single document per element and then run $group to count elements:

db.collection.aggregate([
    {
        $unwind: "$elements"
    },
    {
        $group: {
            _id: "$elements",
            count: { $sum: 1 }
        }
    }
])

EDIT: you can use additional group with $replaceRoot and $arrayToObject to return your ids as keys and counts as values:

db.collection.aggregate([
    {
        $unwind: "$elements"
    },
    {
        $group: {
            _id: "$elements",
            count: { $sum: 1 }
        }
    },
    {
        $group: {
            _id: null,
            counts: { $push: { k: "$_id", v: "$count" } }
        }
    },
    {
        $replaceRoot: {
            newRoot: { $arrayToObject: "$counts" }
        }
    }
])

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answered Dec 15, 2019 at 23:04

mickl

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