title: makeReferenceObject in harmonize flow - how to use the object reference?
body: <p>In the DHF tutorial on harmonizing products and orders, products are included into orders by reference using SKU:</p>
  
  <p><a href="https://marklogic.github.io/marklogic-data-hub/tutorial/harmonizing-order-data/" rel="nofollow noreferrer">https://marklogic.github.io/marklogic-data-hub/tutorial/harmonizing-order-data/</a></p>
  
  <pre><code>// or a reference to a Product
  products.push(makeReferenceObject('Product', order.sku));
  
  ...
  
  function makeReferenceObject(type, ref) {
    return {
      '$type': type,
      '$ref': ref
    };
  }
  </code></pre>
  
  <p>An example of the harmonized order record:</p>
  
  <pre><code>"Order": {
    "id": 1,
    "products": [
      { "Product": {
        "$type": "Product", 
        "$ref": "12345"
      }},
      { "Product": {
        "$type": "Product", 
        "$ref": "54321"
      }}
    ],
    "price": 100
  }
  </code></pre>
  
  <p>Now how can we make use of the product object reference in the order record? Should the <code>entity service</code> built-in function such as <code>es.instanceJsonFromDocument</code> be able to resolve the references and incorporate the product information into the output? Or it is just a reference for developers to implement their own custom logic?</p>