How to 'join' two indexes in ElasticSearch

As answered in your other question, nothing prevents you from storing the Customer name inside each order_item document at indexing time, while still having a dedicated index orders also containing the Customer data. Remember that it's all about cleverly denormalizing your data so that each of your documents be as "self-contained" 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"}

The advantages of this solution is that each order item is completely self-contained, and you can group/aggregate them on OrderID in order to get all items of a given order.

Also, as @JohnAment mentioned in his comment, the order/order_item use case is also a good candidate for using either

1. the parent/child relationship
2. or nested objects.

In the first case, you'd have one order "parent" document...

curl -XPUT localhost:9200/orders/order/82 -d '{    "ID": 82,    "Customer": "John Smith"}'

And several order_item "children" documents that you index using their parent ID:

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}'

In the second case, your order document would contain all order items in a nested OrderItems property and would look like this:

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      }    ]}'