How to Get String Length in MongoDB

In MongoDB, determining the length of a string stored in a document can be useful for various data processing tasks and queries. MongoDB provides several operators and methods that allow us to calculate the length of strings efficiently.

In this article, we will explore how to get the string length in MongoDB by covering concepts and examples in detail that will help us to manipulate and query string data in MongoDB.

How to Get String Length in MongoDB

In MongoDB, string length refers to the number of characters in a string field stored within a document. This length can be calculated dynamically during queries to perform data validation, filtering, or aggregation tasks.

Below are the approaches that help us to understand How to calculate the string length in Mongodb as follows:

1. Using $strLenCP Operator
   
2. Using $expr and $size Operators

Let's set up an Environment:

To understand How to Get String Length in MongoDB we need a collection and some documents on which we will perform various operations and queries. Here we will consider a collection called products which contains the information shown below:

[
  {
    _id: ObjectId('662609de9fe21c773b8bf20c'),
    name: 'Smartphone',
    price: 599
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20d'),
    name: 'Laptop',
    price: 1299
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20e'),
    name: 'Headphones',
    price: 99
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20f'),
    name: 'Tablet',
    price: 399
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf210'),
    name: 'Smartwatch',
    price: 199
  }
]

1. Using $strLenCP Operator

The $strLenCP operator in MongoDB is used to calculate the length of a string in code points. It returns the number of UTF-8 code points in the specified string. This operator is useful for calculating the length of string fields in documents and allowing for various data processing and querying tasks.

Example: Calculating String Length with $strLenCP

To calculate the length of the name field using the $strLenCP operator, use the following query in the MongoDB shell:

// Calculate string length using $strLenCP
db.products.aggregate([
  {
    $project: {
      name: 1,
      nameLength: { $strLenCP: "$name" }
    }
  }
]);

Output:

[
  {
    _id: ObjectId('662609de9fe21c773b8bf20c'),
    name: 'Smartphone',
    nameLength: 10
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20d'),
    name: 'Laptop',
    nameLength: 6
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20e'),
    name: 'Headphones',
    nameLength: 10
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20f'),
    name: 'Tablet',
    nameLength: 6
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf210'),
    name: 'Smartwatch',
    nameLength: 10
  }
]

This MongoDB aggregation query uses the $strLenCP operator to calculate the length of the name field in each document of the products collection. The $project stage is used to include the original name field along with a new nameLength field containing the calculated length

2. Using $expr and $size Operators

The $expr operator allows the use of aggregation expressions inside a $match, $redact, $project, $addFields, and $group stage. It enables the aggregation pipeline to compare fields from the same document or perform other logical expressions.

The $size operator returns the number of elements in an array. It is commonly used in aggregation pipelines to calculate the length of an array field within a document. If the array is null or missing, $size returns 0.

Example: Calculating String Length with $expr and $size

To calculate the length of the interests array using $expr and $size, use the following query:

// Calculate array length using $expr and $size
db.users.aggregate([
  {
    $project: {
      name: 1,
      interestsCount: {
        $size: {
          $ifNull: ["$interests", []]
        }
      }
    }
  }
]);

Output:

[
  {
    _id: ObjectId('662609de9fe21c773b8bf20c'),
    name: 'Smartphone',
    featuresCount: 0
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20d'),
    name: 'Laptop',
    featuresCount: 0
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20e'),
    name: 'Headphones',
    featuresCount: 0
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf20f'),
    name: 'Tablet',
    featuresCount: 0
  },
  {
    _id: ObjectId('662609de9fe21c773b8bf210'),
    name: 'Smartwatch',
    featuresCount: 0
  }
]

This MongoDB aggregation query calculates the length of the interests array field for each document in the users collection. It uses the $size operator within a $project stage to determine the array length, and the $ifNull operator to handle cases where the interests field may be missing or null, assigning an empty array [] in such cases.

Conclusion

Overall, Calculating string length in MongoDB is essential for various data processing and querying tasks. By understanding the operators like $strLenCP, $expr, and $size, you can efficiently determine the length of strings stored in documents or arrays within collections.