filter unique document basis on a particular key lie on document

Question

I am using Mongo aggregate framework, suppose if i am having collection structure like this

{
  {
    _id: ObjectId(123)
    name: john,
    age: 30
  },
  {
    _id: ObjectId(456)
    name: moore,
    age: 45
  },
  {
    _id: ObjectId(789)
    name: carl,
    age: 30
  }
}

I want to get only unique age documents for example, result should look like this

{
  {
    _id: ObjectId(123)
    name: john,
    age: 30
  },
  {
    _id: ObjectId(456)
    name: moore,
    age: 45
  },
}

Above result have removed same age document and taken any single document of them.

How can I perform this set operation on collection on the basis of age, by which I only get unique age document collection using mongo aggregation

Answer 1

db.collection.aggregate([
  {
    $group: {
      _id: "$age",
      firstDocument: {
        $first: "$$ROOT"
      }
    }
  },
  {
    $replaceRoot: {
      "newRoot": "$firstDocument"
    }
  }
])

1. $group will retain single document for age. Since, you want to retain the structure of the document, use $$ROOT to assign the whole document to a field.

   The output of this stage will be something like this.

   {
     "_id": 45,
     "firstDocument": {
       "_id": 345,
       "age": 45,
       "name": "moore"
     }
   }
   

2. $replaceRoot will replace the whole document from previous stage with the one you want. In this case, it is what we assigned to "firstDocument" field.

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