Mongodb count occurences of an element inside the array from group

Question

I have a problem where I want to get the number of occurences of a certain element in an array as i group them. Here is my schema:

{
   "name": "John Doe",
   "age": 20,
   "address": "Nasipit, Talamban, Cebu City, Cebu",
   "visitors": [
     {
       "_id": {
         "$oid": "5de5a8271a91ca42fc5fc593"
       },
       "firstname": "Jane Doe",
       "lastname": "Tanilon",
       "address": "California",
       "date": "December 3rd 2019, 8:11:19 am"
     },
     {
       "_id": {
         "$oid": "5de5a8271a91ca42fc5fc593"
       },
       "firstname": "Nice",
       "lastname": "One",
       "address": "California",
       "date": "December 3rd 2019, 8:11:19 am"
     }
     ]
}

I tried making to group them:

db.visitors.aggregate({
    $group: {_id: '$visitors.address',count:{$sum:1}}
})

And the result is :

{ "_id" : [ "California", "California"], "count" : 1 }

And I want to count occurences of California in the array like this one:

{ "_id" : "California, "count" : 2 }

mickl · Accepted Answer · 2019-12-03 01:52:53Z

3

Expression '$visitors.address' returns an array while you need to count visitors array documents separately. To do that you need to $unwind that array before running $group:

db.visitors.aggregate([
    { $unwind: "$visitors" },
    {
        $group: {_id: '$visitors.address',count:{$sum:1}}
    }
])

answered Dec 3, 2019 at 1:52

mickl

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1 Comment

Chervin Tanilon Over a year ago

I never used $unwind before since I am new to MongoDB, anyways thank you for saving my life!

prasad_ · Accepted Answer · 2019-12-03 03:56:07Z

The following aggregation query counts all the occurances of "California" in all documents.

var INPUT_PARAM = "California"

db.test.aggregate( [
{ 
  $unwind: "$visitors" 
},
{ 
  $group: { 
      _id: null, 
      count: { 
          $sum: { 
              "$cond": [ 
                  { "$eq": [ "$visitors.address", INPUT_PARAM ] }, 
                    1, 
                    0 
              ] 
          } 
      } 
  } 
},
{ 
  $project: { 
      _id: INPUT_PARAM, 
      count: 1 
  } 
}
] )

The result is: { "count" : 3, "_id" : "California" }, when the input documents are:

{ _id: 11,
   "name": "John Doe",
   "address": "Nasipit, Talamban, Cebu City, Cebu",
   "visitors": [
     {
       "_id": 1,
       "firstname": "Jane Doe",
       "address": "California",
     },
     {
       "_id": 2,
       "firstname": "Nice",
       "address": "California",
     },
       {"_id": 3,
       "firstname": "Jack",
       "address": "Texas",
     }
    ]
},
{ _id: 22,
   "name": "Miller",
   "address": "14, North 5th St., New York",
   "visitors": [
     {
       "_id": 95,
       "firstname": "ABC",
       "address": "California",
     },
       { "_id": 96,
       "firstname": "XYZ",
       "address": "Maine",
     }
    ]
}

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