Creating (POST) an entity via a navigation property?

Question

Hi All,
Let's say we have a model that has an Employees entityset, and for a given employee, you can navigate to a CurrentCarParkingSpot entity (it's an example not reality).
e.g. Employees('00123456')/CurrentCarParkingSpot would pull up the current CarParkingSpot against the Employee.
Now if the employee didn't have a CarParkingSpot, and I want to create a new CarParkingSpot for this employee, you would think I could POST a standard CarParkingSpot payload to Employees('00123456')/CurrentCarParkingSpot and it should create a new CarParkingSpot against employee (assuming I get the employee key in the GW code). However, when I try this, the framework doesn't even call my code, and it's almost like Gateway does not support creation via a navigation property. PUT works, just not POST.
Note - If I post the same payload directly to /CarParkingSpots, it works fine.
e.g. The payload looks like:
<?xml version="1.0" encoding="utf-8"?>
<entry xml:base="http://server.local/sap/opu/odata/sap/ZEXAMPLE/" xmlns="http://www.w3.org/2005/Atom" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices">
  <id>http://server.local/sap/opu/odata/sap/ZEXAMPLE/CarParkingSpot</id>
  <title type="text">CarParkingSpot</title>
  <category term="ZEXAMPLE.CarParkingSpot" scheme="http://schemas.microsoft.com/ado/2007/08/dataservices/scheme"/>
  <content type="application/xml">
   <m:properties>
   </m:properties>
  </content>
</entry>
So the key question: Is creation (POST) via Navigation Properties supported in GW, and if it is, do you need to alter the payload at all (note - I've tried changing the id and text above to align with navigation property)?
FYI - I've been testing via Gateway Client, and getting successful creation by using "Use as request" from an existing specific Employees('another id')/CurrentCarParkingSpot and removing all id's in the POST payload.
Thanks,
Matt

Answer 1

Hi Matt,
This is a limitation of GW, not OData. If you peruse some non-SAP OData resources you will find mention of posting to navigations.
The OData 4.0 spec states:
To create an entity in a collection, the client sends a POST request to that collection's URL. The POST body MUST contain a single valid entity representation.
An entity may also be created as the result of an Upsert operation.
If the target URL for the collection is a navigation link, the new entity is automatically linked to the entity containing the navigation link.
The GW framework requires you to POST to a canonical URI, i.e. unnavigated direct address.
In my view navigated POST should be supported by GW otherwise it imposes unrealistic design issues, e.g. you now have to include the key of a parent into a child entity to satisfy your requirement.
There are other reasons why it's desirable to POST to the navigation endpoint.
1 - As the navigation points get longer, the need to add more key properties to the endpoint arises:
e.g. for myservice/entityset1(1)/entityset2(2)/siblings(3)/classifications, the classifications entity needs three properties that probably aren't anything like the canonical URI.
This will, at code level mean there are going to be divergences in the access methods that don't need to be present.
2 - The canonical URI may not be addressable. In the above example, classifications is not likely to be something that has any context outside of a navigated path,so a canonical URI for posting one cannot be established.
I don't know why SAP haven't allowed this as the runtime components certainly have the potential to expose the navigation context if the POST request was allowed in.
In the interim, it might be possible design a model that would allow pseudo-navigation posting by using a batched request, so I may have a look into this.
Regards
Ron.