How to group nested fields in mongodb aggregation?

@ryleigh 

In MongoDB aggregation, you can group nested fields by using the $group stage with the dot notation to access nested fields. Here's an example of how you can group nested fields in MongoDB aggregation:

Suppose you have a collection called "orders" with documents containing nested fields like this:

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{
  "_id": 1,
  "customer": {
    "name": "John Doe",
    "city": "New York"
  },
  "total_amount": 100
}

To group by the "customer.city" field and calculate the total amount for each city, you can use the following aggregation pipeline:

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db.orders.aggregate([
  { $group: {
    _id: "$customer.city",
    total: { $sum: "$total_amount" }
  }}
])

In this aggregation pipeline:

• The $group stage groups the documents by the "customer.city" field using the dot notation.
• The _id field specifies the field by which to group the documents.
• The total field uses the $sum operator to calculate the total amount for each city.

After running this aggregation pipeline, you will get the total amount grouped by the "customer.city" field.