Querying for Documents with Non-Empty Arrays in MongoDB

Finding Documents with Non-Empty Arrays in MongoDB

MongoDB is a flexible document database, and array fields are a common way to store lists of data within documents. A frequent task is to retrieve documents where an array field exists and contains at least one element. This tutorial explains how to efficiently query for such documents using MongoDB’s query operators.

Understanding the Challenge

When querying for documents with non-empty arrays, a naive approach might involve checking for the array’s existence and then verifying its length. However, directly checking the length within a query can be inefficient, especially on large datasets, as it might prevent MongoDB from utilizing indexes effectively. The key is to use operators designed for array comparisons or existence checks.

Querying for Non-Empty Arrays

Here are several methods to query for documents where an array field is not empty:

1. Using $exists and $not with $size

This approach explicitly checks if the array field exists and, if it does, verifies that its size is not zero.

ME.find({
  pictures: {
    $exists: true,
    $not: { $size: 0 }
  }
})

• $exists: true: Ensures that the pictures field exists in the document.
• $not: { $size: 0 }: Checks if the size (number of elements) of the pictures array is not equal to 0.

While this works, $size can sometimes hinder index usage.

2. Using $exists and $ne with an Empty Array

A more efficient and often preferred method is to check if the array exists and is not equal to an empty array ([]).

ME.find({
  pictures: {
    $exists: true,
    $ne: []
  }
})

• $exists: true: Checks if the pictures field exists.
• $ne: []: Checks if the pictures array is not equal to an empty array. This allows MongoDB to use indexes more effectively.

3. Checking for the Existence of the First Element

For a simple existence check, you can query for the presence of the first element in the array:

ME.find({ 'pictures.0': { $exists: true } })

This query checks if the element at index 0 of the pictures array exists. If it does, it implies that the array is not empty. However, this method only confirms that at least one element exists and doesn’t guarantee the array’s overall size.

4. Considering Array Type (For Robustness)

If your pictures field might contain invalid data types (e.g., null, a boolean, or a string instead of an array), it’s a good practice to add a type check for extra robustness.

• MongoDB 3.2 and later:

  ME.find({
    pictures: {
      $exists: true,
      $type: 'array',
      $ne: []
    }
  })
  

• MongoDB before 3.2:

  ME.find({
    pictures: {
      $exists: true,
      $type: 4,
      $ne: []
    }
  })
  

  Here, $type: 'array' (or $type: 4) ensures that the pictures field is actually an array before performing the non-empty check.

Combining with Other Queries

You can combine these array checks with other query criteria using MongoDB’s standard query operators ($and, $or, etc.) to create more complex and specific queries. For example, to find the newest 10 documents with non-empty pictures arrays:

ME.find({
  pictures: {
    $exists: true,
    $ne: []
  }
}).sort({ created: -1 }).limit(10)

This query finds documents where pictures is not empty, sorts them by the created field in descending order (newest first), and limits the results to the top 10 documents.

Choosing the Right Approach

The most efficient approach depends on your data and query patterns:

• For simple non-empty checks where you’re confident about the data type of the array field, $exists and $ne: [] is often the best choice.
• If data type consistency is a concern, add the $type check for added robustness.
• If you only need to confirm the existence of at least one element, checking for the first element can be a lightweight solution. However, it’s less versatile than the other methods.