MongoDB Aggregation $first Operator

The $first operator in MongoDB returns the first document’s field value from each group in an aggregation pipeline, based on the current sort order.

Used within the $group stage of the aggregation pipeline.
Returns the value of a specified field from the first document in each group.
Depends on the input order, so commonly paired with $sort before $group.
Useful for extracting representative or summary values per group.

Syntax

{
  $group: {
    _id: <expression>,
    firstField: { $first: "$fieldName" }
  }
}

_id: The field or expression used to group documents.
firstField: The output field that will contain the first document from each group.
$first: "$fieldName": The $first operator returns the value of the specified field from the first document in each group.

Example of MongoDB Aggregation $first Operator

Consider a collection called students which contains information in various documents are shown below.

([
...   { "name": "Alex", "age": 18, "grade": 10 },
...   { "name": "Ben", "age": 17, "grade": 10 },
...   { "name": "Clevin", "age": 16, "grade": 11 },
...   { "name": "David", "age": 18, "grade": 11 },
...   { "name": "Eva", "age": 17, "grade": 12 },
...   { "name": "Frank", "age": 16, "grade": 12 }
... ])

Example 1: Find the oldest student in each grade after sorting

db.students.aggregate([
  { $sort: { grade: 1, age: -1 } },
  {
    $group: {
      _id: "$grade",
      oldestStudent: { $first: "$name" }
    }
  }
])

Output:

[
  { "_id": 10, "oldestStudent": "Alex" },
  { "_id": 11, "oldestStudent": "Clevin" },
  { "_id": 12, "oldestStudent": "Eva" }
]

$group groups documents by the grade field.
$first: "$name" returns the name of the first document in each group after sorting.
Since documents are sorted by age in descending order within each grade, the first document represents the oldest student.
The result shows the oldest student in each grade.

Example 2: Handling Multiple Fields

Find the first and last names of the oldest student in each grade.

db.students.aggregate([
  {
    $group: {
      _id: "$grade",
      oldestStudentFirst: { $first: "$name" },
      oldestStudentLast: { $last: "$name" }
    }
  }
])