jpa useful links

1. https://vladmihalcea.com/the-best-way-to-map-a-onetomany-association-with-jpa-and-hibernate/
2. https://www.thoughts-on-java.org/best-practices-many-one-one-many-associations-mappings/
3. http://websystique.com/hibernate/hibernate-many-to-one-unidirectional-annotation-example/
4.