Picking first document from a group

Since a group collapses multiple documents into a single document, you must need an accumulator like $first to select the document. This accumulator picks the first document from the group. In the example below, the name is selected based on the first document in the group. $name points to the name property of the selected document.

// test data
db.getCollection("students").insertMany([
	{ name: "Student1", class: "A", score: 92 },
	{ name: "Student2", class: "A", score: 74 },
	{ name: "Student3", class: "B", score: 85 },
	{ name: "Student4", class: "B", score: 61 },
	{ name: "Student5", class: "A", score: 48 },
]);

db.getCollection("students").aggregate([
	{
		$facet: {
			countByClass: [
				{
					$group: {
						_id: "$class",
						count: { $sum: 1 },
						firstStudentName: { $first: "$name" },
					},
				},
				{ $sort: { _id: 1 } },
			],
		},
	},
]);

Output:

[
	{
		"countByClass": [
			{
				"_id": "A",
				"count": 3,
				"firstStudentName": "Student1"
			},
			{
				"_id": "B",
				"count": 2,
				"firstStudentName": "Student3"
			}
		]
	}
]