Is it possible to rename _id field after mongo's group aggregation?

Is it possible to rename _id field after mongo's group aggregation?

In MongoDB's aggregation framework, it is possible to rename the _id field after performing a $group aggregation using the $project stage. You can use $project to reshape the output document and assign a new name to the _id field or any other fields.

Here's an example of how to rename the _id field after a $group aggregation:

Suppose you have a collection of documents like this:

[ { "_id": "A", "count": 3 }, { "_id": "B", "count": 2 }, { "_id": "A", "count": 5 } ]

And you want to group them by the _id field and sum the count field for each group. After that, you want to rename the _id field to group and keep the total field:

db.collection.aggregate([ { $group: { _id: "$_id", total: { $sum: "$count" } } }, { $project: { group: "$_id", // Rename _id to group total: 1, // Keep the total field _id: 0 // Exclude the _id field if you don't need it } } ]);

The $project stage is used to reshape the document:

  • group: "$_id" renames the _id field to group.
  • total: 1 keeps the total field.
  • _id: 0 excludes the _id field from the output if you don't need it. You can omit this line if you want to keep the _id field in the result.

The output of this aggregation will look like:

[ { "group": "A", "total": 8 }, { "group": "B", "total": 2 } ]

As you can see, the _id field has been renamed to group, and you have the total field in the result.

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