MongoDB - $isArray Operator

In MongoDB, arrays are used to store lists of information such as product categories or tags. The $isArray operator is a tool that helps us to check if a specific field contains an array. This is important for managing data effectively especially when dealing with mixed data types in our collections.

In this article, We will learn about the MongoDB $isArray Operator by understanding various examples, their working and so on.

What is the MongoDB $isArray Operator?

In MongoDB, arrays are commonly used to store lists of data, such as lists of products, tags or items in stock. However, sometimes we need to verify whether a specific field contains an array or another data type like a string or number.

The $isArray operator in MongoDB is designed for the purpose which check whether the provided expression evaluates to an array and returns a boolean value (true or false). The $isArray operator in MongoDB is used to determine if a given field or expression is an array. It returns a boolean value:

• true if the expression evaluates to an array.
• false if the expression does not evaluate to an array.

Syntax:

{ $isArray: [ <expression> ] }

• Expression: This can be any valid MongoDB expression, such as a field or a computed value.

The $isArray operator is used in aggregation pipelines where determining whether a field is an array is necessary for further processing. It is particularly useful when we have a collection where some fields can hold either an array or a scalar value, and we want to conditionally apply operations based on the field type.

When to Use the $isArray Operator?

The $isArray operator is useful in scenarios where:

• We have fields in our documents that could be either arrays or non-array types.
  
• We want to perform operations (such as counting array elements, adding items to an array, or performing conditional updates) based on whether a field is an array or not.
  
• By checking if a field is an array, we can write more efficient and dynamic queries that adapt to different data types in your collection.

Examples of MongoDB $isArray Operator

Let's consider below collection called products on which we will perform the various examples and so on.

db.products.insertMany([
   { _id: 1, name: "Laptop", categories: ["Electronics", "Computers"], stock: 30 },
   { _id: 2, name: "Phone", categories: "Electronics", stock: 100 },
   { _id: 3, name: "Shirt", categories: ["Apparel", "Men"], stock: 50 },
   { _id: 4, name: "Shoes", categories: ["Apparel", "Women"], stock: 25 },
   { _id: 5, name: "Watch", stock: 15 }
])

Example 1: Basic $isArray Usage

Suppose we have a collection of products, and each product can have a categories field that may either be an array or a string. We want to check if the categories field is an array for each product.

Query:

db.products.aggregate([
   {
      $project: {
         name: 1,
         isCategoriesArray: { $isArray: "$categories" }
      }
   }
])

Output:

[
  { "_id": 1, "name": "Laptop", "isCategoriesArray": true },
  { "_id": 2, "name": "Phone", "isCategoriesArray": false },
  { "_id": 3, "name": "Shirt", "isCategoriesArray": true },
  { "_id": 4, "name": "Shoes", "isCategoriesArray": true },
  { "_id": 5, "name": "Watch", "isCategoriesArray": false }
]

Explanation:

The isCategoriesArray field checks whether the categories field is an array. For Laptop, Shirt, and Shoes, this field is true, while for Phone and Watch, it is false because either the value is a string or the field does not exist.

Example 2: Conditionally Applying Logic Based on Array Check

In this example, we’ll check if categories is an array and concatenate the array with a new value only if it is an array. If it’s not an array, we will return "Not an array".

Query:

db.products.aggregate([
   {
      $project: {
         name: 1,
         updatedCategories: {
            $cond: {
               if: { $isArray: "$categories" },
               then: { $concatArrays: [ "$categories", [ "Discounted" ] ] },
               else: "Not an array"
            }
         }
      }
   }
])

Output:

[
  { "_id": 1, "name": "Laptop", "updatedCategories": [ "Electronics", "Computers", "Discounted" ] },
  { "_id": 2, "name": "Phone", "updatedCategories": "Not an array" },
  { "_id": 3, "name": "Shirt", "updatedCategories": [ "Apparel", "Men", "Discounted" ] },
  { "_id": 4, "name": "Shoes", "updatedCategories": [ "Apparel", "Women", "Discounted" ] },
  { "_id": 5, "name": "Watch", "updatedCategories": "Not an array" }
]

Explanation:

This query checks if categories is an array. If it is, the operator concatenates the existing array with ["Discounted"]. If it’s not an array (like for Phone or Watch), it returns "Not an array".

Example 3: Count Elements in an Array

Let’s say we want to count how many categories each product has. We can use $isArray to check if categories is an array and then apply the $size operator to count the number of elements.

Query:

db.products.aggregate([
   {
      $project: {
         name: 1,
         categoryCount: {
            $cond: {
               if: { $isArray: "$categories" },
               then: { $size: "$categories" },
               else: 0
            }
         }
      }
   }
])

Output:

[
  { "_id": 1, "name": "Laptop", "categoryCount": 2 },
  { "_id": 2, "name": "Phone", "categoryCount": 0 },
  { "_id": 3, "name": "Shirt", "categoryCount": 2 },
  { "_id": 4, "name": "Shoes", "categoryCount": 2 },
  { "_id": 5, "name": "Watch", "categoryCount": 0 }
]

Explanation:

• This query Checks if categories is an array.
• If it is, it counts the number of items using the $size operator.
• If it’s not an array, it assigns 0 to categoryCount.

Example 4: Filtering Documents Where a Field Is an Array

If we need to filter out only those documents where the categories field is an array, you can use $isArray in combination with $match.

Query:

db.products.aggregate([
   {
      $match: {
         $expr: { $isArray: "$categories" }
      }
   }
])

Output:

[
  { "_id": 1, "name": "Laptop", "categories": [ "Electronics", "Computers" ], "stock": 30 },
  { "_id": 3, "name": "Shirt", "categories": [ "Apparel", "Men" ], "stock": 50 },
  { "_id": 4, "name": "Shoes", "categories": [ "Apparel", "Women" ], "stock": 25 }
]

Explanation:

Here, we are using $match with $expr to filter documents where the categories field is an array. As a result, the query returns only the documents for Laptop, Shirt, and Shoes.

Best Practices for Using the $isArray Operator

1. Use $isArray with Conditional Operations: Use $isArray within the $cond operator or other conditional operators to perform different operations based on the data type.
   
2. Efficient Data Handling: When working with collections where fields can vary in type, $isArray helps ensure that only the appropriate operations are applied to array fields.
   
3. Combine with Other Operators: Use $isArray with operators like $size, $concatArrays, or $push to manipulate array fields dynamicall

Conclusion

The MongoDB $isArray operator is a powerful tool that allows you to check whether a specific field in your document is an array. It helps us manage and manipulate data efficiently, especially when working with collections that contain mixed data types. By incorporating $isArray in our aggregation pipelines, we can write more flexible and dynamic queries that improve data processing and query efficiency.