Return array length instead of elements in MongoDB query

Question

I want to be able to specify in my MongoDB query to, for each document, return the number of elements in a specific sub field (var_2 in this case), instead of the whole array. Example document:

{
  "_id": "abc123",
  "var_1": "A",
  "var_2": [
    "A", 
    "B", 
    "C"
  ]
}

I have tried this but it returns the whole array:

db.collection.find({var_1: "A"}, {var_1: 1, var_2: 1})

Desired output:

{
"_id": "abc123",
"var_1": "A",
"var_2": 3
}

Thanks in advance!

Ayoub · Accepted Answer · 2020-01-11 14:51:38Z

1

You can retrieve the size of your array by using $size but with an aggregation since it's not supported in find' projection:

db.your_collection.aggregate([
  {
    "$match": {
      var_1: "A"
    }
  },
  {
    "$project": {
      "var_1": 1,
      "var_2": {
        "$size": "$var_2"
      }
    }
  }
])

Result

[
  {
    "_id": "abc123",
    "var_1": "A",
    "var_2": 3
  }
]

answered Jan 11, 2020 at 14:51

Ayoub

1,43514 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Return array length instead of elements in MongoDB query

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related