Combine aggregations from two nested objects

[esbenp (Esben Petersen)]

How can I create an combined aggregation in Elasticsearch using multiple nested objects?

What I want to achieve is the SUM of quantity grouped by country in a mapping that looks like this

curl -XPUT 'localhost:9200' -d '{
    "index": "test",
    "type": "orders",
    "body": {
        "orders": {
            "order_number": {
                "type": "string",
                "index": "not_analyzed"
            },
            "buyer": {
                "type": "nested",
                "properties": {
                    "country": {
                        "type": "string",
                        "index": "not_analyzed"
                    }
                }
            },
            "products": {
                "type": "nested",
                "properties": {
                    "name": {
                        "type": "string",
                        "index": "not_analyzed"
                    },
                    "quantity": {
                        "type": "string",
                        "index": "not_analyzed"
                    }
                }
            }
        }
    }
}'

Using this test data

curl -XPOST 'localhost:9200/_bulk' -d '
    {"index": {"_index": "test", "_type": "orders", "_id": 1}}
    {"order_number": "1", "buyer": {"country": "US"}, "products": [{"name": "A product", "quantity": 10}]}
    {"index": {"_index": "test", "_type": "orders", "_id": 2}}
    {"order_number": "2", "buyer": {"country": "DE"}, "products": [{"name": "A product", "quantity": 10}]}
    {"index": {"_index": "test", "_type": "orders", "_id": 3}}
    {"order_number": "3", "buyer": {"country": "US"}, "products": [{"name": "A product", "quantity": 10}]}
'

The result should be that there was sold 10 "A product" in DE and 20 in US. How can I create this aggregation when the properties in question are on separate nested objects?