Question

How do you create a composite _id field in the $group stage?

Answer 1

To create a composite _id field in the $group stage, one should specify an object as the value of the _id field in the MongoDB aggregation pipeline, with each field in the object representing part of the composite key.

Step-by-step explanation:

To create a composite _id field in the $group stage of an aggregation pipeline in MongoDB, you would specify an object as the value of the _id field that contains the fields you want to group by. Each field in this object represents a component of the composite key.

Here is an example of how to group documents by combining two fields, fieldA and fieldB, into a composite _id:

{
$group: {
_id: { fieldA: "$fieldA", fieldB: "$fieldB" },
total: { $sum: 1 }
}
}

In this example, the result of the $group stage will output documents where each unique combination of fieldA and fieldB will be represented by a single document with the sum total of occurrences as an additional field.