What does Spring Data Couchbase use the _class field for?

Question

I'm guessing that the type is used for CRUD operations. Is it used for anything else besides that? I'm wondering what impact there could be from configuring how it gets populated.

Answer 1

The _class field is written to allow polymorphic properties in your domain model. See this sample:

class Wrapper {
  Object inner;
}

Wrapper wrapper = new Wrapper();
wrapper.inner = new Foo();

couchbaseOperations.save(wrapper);

You see how the field inner will get Foo serialized and persisted. On the reading side of things we now have to find out which type to create an object of and the type information in Wrapper is not enough to do so as it only states Object.

That's why Spring Data object mapping persists an additional field (name customizable but defaulting to _class) to store that information to be able to inspect the source document, derive a type from the value written for that field and eventually map that document back to that particular type.

The Spring Data Couchbase reference documentation doesn't really document it, you can find information about the way this works in the docs for the MongoDB module. I've also created a ticket for Spring Data Couchbase to improve on the docs for that.