As pointed out in another question , nothing prevents you from storing the name Customerinside each document order_itemduring indexing, but it also has a dedicated index orders, which also contains data Customer. Remember that all this is connected with the mental denormalization of your data, so that each of your documents is "self-sufficient" as you need.
curl -XPUT localhost:9200/order_items/order_item/1 -d '{
    "ID": 1,
    "Name": "Shoes",
    "Price": 9.99,
    "OrderID": 82,
    "Customer": "John Smith"
}'
curl -XPUT localhost:9200/order_items/order_item/2 -d '{
    "ID": 2,
    "Name": "Hat",
    "Price": 19.99,
    "OrderID": 82,
    "Customer": "John Smith"
}
, , / OrderID, .
, @JohnAment , order/order_item
order...
curl -XPUT localhost:9200/orders/order/82 -d '{
    "ID": 82,
    "Customer": "John Smith"
}'
order_item "children", , :
curl -XPUT localhost:9200/order_items/order_item/1?parent=82 -d '{
     "ID": 1,
     "Name": "Shoes",
     "Price": 9.99
}'
curl -XPUT localhost:9200/order_items/order_item/2?parent=82 -d '{
     "ID": 2,
     "Name": "Hat",
     "Price": 19.99
}'
order OrderItems :
curl -XPUT localhost:9200/orders/order/82 -d '{
    "ID": 82,
    "Customer": "John Smith"
    "OrderItems": [
      {
        "ID": 1,
        "Name": "Shoes",
        "Price": 9.99
      },{
        "ID": 2,
        "Name": "Hat",
        "Price": 19.99
      }
    ]
}'