Hibernate creating redundant many-to-many tables

Question

While developing my Spring Boot application I had to drop my database and have Hibernate generate it again with hibernate.hbm2ddl.auto=update. After that I want

Answer 1

If you use @OneToMany without @JoinColumn Hibernate creates a join table

@OneToMany(fetch = FetchType.EAGER, targetEntity = Skill.class)
@Cascade({CascadeType.DETACH})
private Set<Skill> children;

change it to

@OneToMany(fetch = FetchType.EAGER)
@Cascade({CascadeType.DETACH})
@JoinColumn
private Set<Skill> children;

Please, don't use unnecessary mapping properties — targetEntity = Skill.class

And I am not sure you need to use this association

private Skill parent; 

It is incorrect to associate User and Role using the @OneToMany association, because of User can have multiple roles and each role can belong to multiple users. Use @ManyToMany for such an association.

An example of an association between User and Role

Answer 2

What you have here is a bidirectional association in which both sides are owners, turning it basically to two independent associations. In a one-to-many association the owner is usually the many-to side (note the mappedBy attribute):

OneToMany(fetch = FetchType.EAGER, targetEntity = Skill.class, mappedBy = "parent")
@Cascade({CascadeType.DETACH})
private Set<Skill> children;

This way Hibernate will ignore one-to side when maintaining the relationship (and will not create the join table which is the default configuration for @OneToMany association without @JoinColumn).