In MongoDB, to aggregate and count nested array elements, we can use $unwind and $group aggregation stages. Bobcares, as a part of our Server Management Service offers solutions to every query that comes our way.

Aggregate and Count Nested Array Elements in MongoDB

The Aggregation Framework in MongoDB offers strong tools for handling data processing tasks on collections. Several operators and stages are provided by the Aggregation Framework for helping with the counting of elements within nested arrays.

1. Unwind the Arrays: To “unwind” the nested arrays is the main idea. This means changing the document structure so that each nested array element appears as a distinct one. For this use, aggregation provides the $unwind operator.

2. Group by the Desired Field: The $group operator can be used to group documents according to the field we wish to count once the nested array has been unwound. This can be a field inside the nested element or the element itself.

3. Count the Elements: We can calculate the overall count using the $sum operator in the $group stage. By default, it counts the number of documents in each group by passing in 1 as its parameter.

An Example

Document:

{
"_id": 1,
"name": "Product A",
"categories": [
"Electronics",
"Gadgets"
]
}

Aggregation Pipeline:

db.collection.aggregate([
{ $unwind: "$categories" }, // Unwind the categories array
{ $group: { _id: "$categories", count: { $sum: 1 } } } // Group by category and count elements
])

Result:

[
{ "_id": "Electronics", "count": 1 },
{ "_id": "Gadgets", "count": 1 }
]

After unwinding the categories array, this pipeline classifies documents according to their category value (_id). In the end, it counts the number of documents in each group (or, more accurately, the number of items that fall within that category) using $sum.

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Conclusion

The article explains more about aggregating & counting nested array elements in MongoDB.