MongoDB: Checking if a Field is Array Using $isArray Operator

Introduction

MongoDB is a highly popular NoSQL database for modern applications that require flexibility, scalability, and high performance. One of its key features includes supporting arrays as field values. In this tutorial, we will explore how to check if a field within a MongoDB document is an array by using the $isArray aggregation operator. Understanding this operator and its proper implementation is crucial to maintain data integrity and facilitate data manipulations involving arrays.

What is the $isArray Operator Used for?

The $isArray operator is introduced in MongoDB as a part of the aggregation framework. This operator checks whether a specified field holds an array, returning a boolean value (true or false). Knowing the type of a field can be particularly useful in conditional logic, data validation, shaping aggregation pipelines, and performing certain update operations effectively.

Basic Usage of $isArray

Let’s start by understanding the basic usage of $isArray through an example. Consider the following document in a collection named inventory:

{
    "_id": ObjectId("5fc9329bdb28d35aeedeabg9"),
    "item": "ABC123",
    "tags": ["clothing", "men", "shirt"]
}

To check if the tags field is an array, we can utilize an aggregation pipeline with a $project stage as follows:

db.inventory.aggregate([
    {
        $project: {
            item: 1,
            tagsIsArray: { $isArray: "$tags" }
        }
    }
]);

This will produce:

{
    "_id": ObjectId("5fc9329bdb28d35aeedeabg9"),
    "item": "ABC123",
    "tagsIsArray": true
}

In the example above, we’ve added a new field tagsIsArray to our projection, specifically to hold the boolean result of the $isArray operator acting upon the tags field.

Comparing Field Values Against Arrays

Sometimes you might need to ensure one field is an array and compare it with another field. Let’s say our documents look like the following:

{
    '_id': ObjectId("5fc9329fej28d35aeedfre3"),
    'item': 'XYZ456',
    'tags': ['tools', 'hardware'],
    'category': 'tools'
}

Now, we want to find out if the tags field is an array and also whether the category field is included within this array. We can do this through:

db.inventory.aggregate([
    {
        $project: {
            item: 1,
            tagsIsArray: { $isArray: "$tags" },
            categoryIncluded: { $in: ["$category", "$tags"] }
        }
    }
]);

The above aggregation ensures that tags is an array, and then checks whether the category value exists within tags.

Validating Documents with Conditions

Moving towards slightly more advanced scenarios, imagine needing to validate documents where a tags field is required to exist and be an array. You can achieve it using the $match stage combined with the $expr and $isArray:

db.inventory.aggregate([
    {
        $match: {
            $expr: { $isArray: "$tags" }
        }
    },
    {
        $project: {
            item: 1,
            tags: 1
        }
    }
]);

This pipeline filters out documents where tags is not an array before reaching the $project stage, thus ensuring that all resulting documents fulfill our array requirement for the tags field.

Complex Aggregations Involving Arrays

For an even more complex use case, consider a situation where you would need to unwind an array field, perform operations on each item, and then check if another field is also an array:

db.inventory.aggregate([
    {
        $unwind: "$tags"
    },
    {
        $groupBy: {
            _id: "$item",
            allTags: { $push: "$tags" }
        }
    },
    {
        $project: {
            item: 1,
            allTagsIsArray: { $isArray: "$allTags" }
        }
    }
]);

Here, the $unwind stage deconstructs array fields into separate documents for each element, the $groupBy stage aggregates them back with specific logic (if any), and the $project stage applies the $isArray to confirm the reconstructed field is an array.

Using $isArray in Update Operations

In some cases, update operations in MongoDB might need to be contingent on whether a field is an array. Here’s an example using the new update with aggregation pipeline feature:

db.inventory.updateMany({}, [
    {
        $set: {
            hasMultipleTags: {
                $cond: {
                    if: { $isArray: "$tags" },
                    then: { $gt: [{ $size: "$tags" }, 1] },
                    else: false
                }
            }
        }
    }
]);

This update operation goes through all documents, checks if tags is an array, and adds a new boolean field hasMultipleTags which notes whether tags contains more than one element.

Conclusion

In this tutorial, we ventured through the basic to advanced usage of MongoDB’s $isArray operator. We highlighted its role within aggregation pipelines, ranging from simple existence checks to complex conditional update operations. Effective use of the $isArray operator augments MongoDB’s power in handling array fields and ensures appropriate data retrieval and manipulation workflows.