`EntityModule` only keeps `@iot.id` of nested entities after JSON serialization

[PhilippMatthes]

Currently, it is not clear enough how the EntityModule intends to serialize entities.

FROST-Client/src/main/java/de/fraunhofer/iosb/ilt/sta/jackson/EntitySerializer.java

Line 58 in 2ff4182

gen.writeFieldName("@iot.id");

FROST-Client/src/main/java/de/fraunhofer/iosb/ilt/sta/jackson/EntitySerializer.java

Line 80 in 2ff4182

gen.writeFieldName("@iot.id");

Due to the above lines, prepopulated fields of subentities are thrown away. I would like to know the reasoning for this design decision to further understand if this is intended behaviour or not. In our use case, we would like to serialize/deserialize a Thing including its Datastreams and Locations. However, the fields of Datastreams and Locations that were populated by an Expand query are thrown away during JSON serialization. Could you maybe elaborate a bit on the above, and point me into the right direction on how I can serialize the subentities without losing their populated fields and not having to perform separate fetch queries?

[hylkevds]

Expanded entities in POSTs should either have an @iot.id, or other fields, but never both:

• When updating an entity (PATCH / PUT) sub-entities are only allowed to be references for updating relations. Only the @iot.id is allowed to be in the related entity in this case.
• When creating entities (POST), related entities must either:
  • not exist yet and thus created using deep-inset, in which case they have no @iot.id, but all other properties
  • already exist and thus linked, in which case only the @iot.id is allowed to be in the json.

hence, sub-entities are never serialised with both their @iot.id and their properties, since such a representation is never valid when being sent to a SensorThings service.

[PhilippMatthes]

Thank you for the elaborate answer. Maybe it should be documented that the EntityModule relates to internal JSON Serialization of the framework when sending requests to the SensorThings service. In my case I just expected that the EntityModule can be reused for the JSON serialization of Things and their nested properties. Hence, an added documentation on the behaviour of the EntityModule, especially EntitySerializer would increase the reusability of the framework.