I fixed this by just moving away from TomEE+ and to Spring Framework (with an embedded
Tomcat Server), which uses Jackson as default instead of "org.apache.johnzon. With this, I was able to use the useful annotations:
This did so the JSON would look like this:
And so alike for User inside Post too, granted they're also marked with those annotations. This is exactly what I wanted and what the front-end web framework I use, EmberJS, and many alike, expect the data to be returned. So this is perfect for me. Also, after playing around with Spring for a little while with an embedded Tomcat server, I like it more too (I've been through Glassfish, TomEE+, and now Spring Framework with embedded Tomcat, and only now is it working as I wanted it to).