Parent-Child and cardinality aggregation

[martingrayson]

I have a parent child relationship setup:

Customer (Parent) -> Order Line (Child)

I'm trying to count the number of parents (customers) that have children matching two sets of criteria. i.e.
I want the number of customers that have purchased both a Foo and a Bar.

At the moment, I can only get vaguely close to this by a complex series of bucketing, just using the child type.

GET customer_orderlines/orderline/_search
{
  "size": 0,
  "query": {
    "bool": {
      "should": [
        {
          "match": {
            "prod_key": "foo"
          }
        },
        {
          "match": {
            "prod_key": "bar"
          }
        }
      ]
    }
  },
  "aggs": {
    "customers": {
      "terms": {
        "field": "customer_key"
      },
      "aggs": {
        "matching_products": {
          "cardinality": {
            "field": "prod_key"
          }
        },
        "sales_bucket_filter": {
          "bucket_selector": {
            "buckets_path": {
              "matchingProducts": "matching_products"
            },
            "script": "params.matchingProducts > 1"
          }
        }
      }
    }
  }
}

Whats the correct way of doing this? Basically im trying to emulate the following sort of SQL

SELECT Count(DISTINCT customer_key) 
FROM   (SELECT customer_key, 
               Count(DISTINCT product) AS prd_count 
        FROM   orders 
        WHERE  orders.product IN ( 'foo', 'bar' )) 
WHERE  prd_count > 1