MongoDB: 3 ways to select a random document from a collection

Introduction

Selecting a random document from a MongoDB collection can be useful in a variety of applications, from displaying random user testimonials on a website to selecting a random item for a game. MongoDB doesn’t have a direct ‘SELECT RANDOM’ command like some SQL databases, but there are several methods to achieve similar functionality. Below are multiple solutions, their implementations, and considerations.

Solution 1: The $sample Aggregation Stage

The $sample stage randomly selects the specified number of documents from its input. It’s the most straightforward and efficient way to select a random document in most cases.

• Step 1: Use the aggregate method on your collection.
• Step 2: Include the $sample stage in your aggregation pipeline, specifying the size as 1 to return a single random document.

Example:

db.collection.aggregate([
  { $sample: { size: 1 } }
])

Notes: This method is performant and concise but may have limitations in large collections or require additional resources from MongoDB, especially when the sample size is significant.

Solution 2: Skip Randomly

Another approach is to generate a random number in your application, use it to skip that many documents in the collection, and then take the next document.

• Step 1: Count the total number of documents.
• Step 2: Generate a random number from 0 to the total number minus one.
• Step 3: Use the find method with skip to skip that many documents and limit the result to 1 document.

Example:

let totalDocs = db.collection.count();
let randomIndex = Math.floor(Math.random() * totalDocs);
db.collection.find().skip(randomIndex).limit(1).toArray();

Notes: This approach can be inefficient on large collections because skip has to traverse all documents until it reaches the specified number. It’s more suitable for relatively small or medium-sized collections.

Solution 3: Random Field and Query

Add a field with a random number to each document. When you need a random document, generate a query with a random number and find the document closest to that number.

• Step 1: Add a random number field to each document.
• Step 2: Generate a random number for the query.
• Step 3: Use the find method with a query to select the document with a random field closest to the generated number.

Example:

db.collection.updateMany({}, [
  {
    $set: {
      randomNumber: {
        $rand: {}
      }
    }
  }
]);

let queryNumber = Math.random();
db.collection.find({ randomNumber: { $gte: queryNumber } }).limit(1).toArray();

Notes: While this method requires some initial setup and ongoing maintenance of the random field, it provides an efficient way to select random documents, especially in large collections with frequent random document fetches.

Conclusion

Selecting a random document in MongoDB can be achieved in several ways, each with its own set of trade-offs regarding performance and complexity. The $sample aggregation stage is generally the simplest and most efficient option, but it may not always be the best choice, depending on the size of the collection and resource availability. The skip method and the random field query provide alternatives that might be more suitable in certain situations. Ultimately, the best approach depends on your specific requirements and constraints.