Correct OneToMany relationship for immutable data classes

[dstepanov]

Fixes #1054

[PiotrBaczkowski]

Definitely my knowledge is not good enough to fix this, but shouldn't kotlin mapping work also with vars and not vals in data classes?

if somebody comes across this issue,
as for now TO ONE relationships has to be vars
TO MANY relationship has to be vals to make it work properly

@graemerocher if its intended then I will be more than happy to update docs about it.

[dstepanov]

It's supposed to work, there are some scenarios like bi-directional relationships that cannot be supported for immutable classes.

Can you please create a PR with your example and add it here https://github.com/micronaut-projects/micronaut-data/tree/master/doc-examples/jdbc-example-kotlin/src/main/kotlin/example

Docs are always welcome but let's first take a look at your case.