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" }
 }
 }
])

Output:

[
 { "_id": 10, "oldestStudentFirst": "Alex", "oldestStudentLast": "Ben" },
 { "_id": 11, "oldestStudentFirst": "Clevin", "oldestStudentLast": "David" },
 { "_id": 12, "oldestStudentFirst": "Eva", "oldestStudentLast": "Frank" }
]

Used $first to retrieve the first name and $last to retrieve the last name of the oldest student in each grade.