how to group array value with count in mongodb?

Question

I've got a collection like this:

{
  "_id": ObjectID(1),
  "fruits": [Banana, Banana, Apple]
}
{
  "_id": ObjectID(2),
  "fruits": [Apple]
}

I want to get the count of the array data, like 2 Banana and 1 Apple for the first object; 1 Apple for the second object.

How can i do this?

Answer 1

1. $unwind - Deconstruct fruits array into multiple documents.

2. $group - Group by _id and fruits. Perform count.

3. $group - Group by _id._id. Push object into fruits array.

4. $set - Set fruits to key-value pair.

db.collection.aggregate([
  {
    $unwind: "$fruits"
  },
  {
    $group: {
      _id: {
        _id: "$_id",
        fruit: "$fruits"
      },
      count: {
        $sum: 1
      }
    }
  },
  {
    $group: {
      _id: "$_id._id",
      fruits: {
        $push: {
          k: "$_id.fruit",
          v: "$count"
        }
      }
    }
  },
  {
    $set: {
      fruits: {
        $arrayToObject: "$fruits"
      }
    }
  }
])

Sample Mongo Playground

Output

[
  {
    "_id": 2,
    "fruits": {
      "Apple": 1
    }
  },
  {
    "_id": 1,
    "fruits": {
      "Apple": 1,
      "Banana": 2
    }
  }
]