[SalesForce] How to access nested JSON (sub-objects) in data

Your data model is analogous in levels to Opportunity - OpportunityLineItem - PricebookEntry - Product2

If you use SFDC Workbench REST Explorer with this query

select id,name,
  (select id, quantity, priceBookEntry.unitPrice, pricebookEntry.product2.productCode
    from OpportunityLineItems)
  from Opportunity where id='xxx'

You'll see how SFDC will JSON serialize the results:

{ "totalSize" : 1, "done" : true, "records" : 
   [ { "attributes" : { 
       "type" : "Opportunity", 
       "url" : "/services/data/v33.0/sobjects/Opportunity/006J000000IDHEhIAP" }, 
       "Id" : "006J000000IDHEhIAP", 
       "Name" : "MyOppoName", 
       "OpportunityLineItems" : { 
         "totalSize" : 1, 
         "done" : true, 
         "records" : [ 
           { "attributes" : { 
              "type" : "OpportunityLineItem", 
              "url" : "/services/data/v33.0/sobjects/OpportunityLineItem/00kJ0000008Tw5nIAC" },
              "Quantity" : 1.0, 
              "PricebookEntry" : { 
                 "attributes" : { 
                    "type" : "PricebookEntry", 
                    "url" : "/services/data/v33.0/sobjects/PricebookEntry/01uJ000000BUzNtIAL" }, 
                    "UnitPrice" : 0.0, 
                    "Product2" : { 
                        "attributes" : { 
                        "type" : "Product2", 
                        "url" : "/services/data/v33.0/sobjects/Product2/01tJ0000004HBbWIAW" }, 
                        "ProductCode" : "MyProductCode" 
     } } } ] 
   } } 
  ] 
 }

If your target system for this JSON happens to also be SFDC, the structure is super convenient as it can be deserialized by the target system directly into sobjects.

However, if your target system has its own JSON structure, you are best of modelling that with inner classes as @susanoochidori suggests. You'll need to do the aforementioned SOQL in your APEX (changing to your custom objects from the OOB example I used) and then, map with assignment statements and inner class instanatiation the values returned from SOQL. Finally, serialize OuterWrapper.

Put another way, you may need to work backwards from the JSON signature expected by the target system into a corresponding APEX outer with inner classes object model

I managed to find a solution here under Nested Datasource Example: https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-programmatic-content.meta/mc-programmatic-content/gtlDatasourceTagExamples.htm

I literally copy/paste the example and got a working version. You can nest the GTL to loop again through your tracking numbers.

Hope this helps!