How to find two random documents in a MongoDB collection of 6?

To find two random documents from a MongoDB collection, use the $sample aggregation stage. This operator randomly selects a specified number of documents from the collection efficiently.

Syntax

db.collection.aggregate([
    { $sample: { size: NumberOfDocuments } }
]);

Sample Data

Let us create a collection with 6 student documents ?

db.twoRandomDocumentDemo.insertMany([
    {"StudentId": 10},
    {"StudentId": 100},
    {"StudentId": 45},
    {"StudentId": 55},
    {"StudentId": 5},
    {"StudentId": 7}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c9ec9aad628fa4220163b87"),
        ObjectId("5c9ec9add628fa4220163b88"),
        ObjectId("5c9ec9b0d628fa4220163b89"),
        ObjectId("5c9ec9b3d628fa4220163b8a"),
        ObjectId("5c9ec9b7d628fa4220163b8b"),
        ObjectId("5c9ec9bad628fa4220163b8c")
    ]
}

Following is the query to display all documents from the collection ?

db.twoRandomDocumentDemo.find();

{ "_id": ObjectId("5c9ec9aad628fa4220163b87"), "StudentId": 10 }
{ "_id": ObjectId("5c9ec9add628fa4220163b88"), "StudentId": 100 }
{ "_id": ObjectId("5c9ec9b0d628fa4220163b89"), "StudentId": 45 }
{ "_id": ObjectId("5c9ec9b3d628fa4220163b8a"), "StudentId": 55 }
{ "_id": ObjectId("5c9ec9b7d628fa4220163b8b"), "StudentId": 5 }
{ "_id": ObjectId("5c9ec9bad628fa4220163b8c"), "StudentId": 7 }

Example: Get 2 Random Documents

Set the size as 2 to get exactly 2 random documents from the collection of 6 ?

db.twoRandomDocumentDemo.aggregate([
    { $sample: { size: 2 } }
]);

{ "_id": ObjectId("5c9ec9b3d628fa4220163b8a"), "StudentId": 55 }
{ "_id": ObjectId("5c9ec9aad628fa4220163b87"), "StudentId": 10 }

Running the Query Again

Each execution returns different random documents ?

db.twoRandomDocumentDemo.aggregate([
    { $sample: { size: 2 } }
]);

{ "_id": ObjectId("5c9ec9add628fa4220163b88"), "StudentId": 100 }
{ "_id": ObjectId("5c9ec9b7d628fa4220163b8b"), "StudentId": 5 }

Key Points

• $sample uses efficient random sampling algorithms for large collections.
• The size parameter specifies how many random documents to return.
• Each query execution produces different random results.

Conclusion

Use $sample in an aggregation pipeline to efficiently select random documents from a collection. This method is optimized for performance and works well with both small and large datasets.