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Yes, the _id field can be renamed after doing a group aggregation in MongoDB by using the $project stage. Here is an example:

db.collection.aggregate([
  { $group: {
      _id: "$category",
      avgRating: { $avg: "$rating" }
  }},
  { $project: {
      category: "$_id",
      avgRating: 1,
      _id: 0
  }}
])

In this example, the group aggregation calculates the average rating for each category in the collection. Then, the $project stage renames the _id field to category and removes it from the output. The resulting output will have a category field instead of an _id field.

Note: It is important to ensure that the renamed field does not conflict with any other field names in the output.