JPA (with Play!) bidirectional Many-to-Many - how to remove from both sides of the relationship?

I have the following mapping in my Play! app using JPA:

@Entity
public class Contact extends Model {        
    public String name;

    @ManyToMany(mappedBy = "contacts", fetch = FetchType.EAGER)
    public Set<Category> categories = new HashSet<Category>();

    public void addCategory(Category c) {
        this.categories.add(c);
        if (!c.contacts.contains(this)) {
            c.contacts.add(this);
        }
    }

    @PreRemove
    public void preRemove() {
        for (Category c : this.categories) {
            c.contacts.remove(this);
        }
        this.categories = null;
    }
}

@Entity
public class Category extends Model {
    public String name;

    @ManyToMany
    public Set<Contact> contacts = new HashSet<Contact>();

    public void addContact(Contact c) {
        this.contacts.add(c);
        if (!c.categories.contains(this)) {
            c.categories.add(this);
        }
    }

    @PreRemove
    protected void preRemove() {
        for (Contact c : this.contacts) {
            c.categories.remove(this);
        }
        this.contacts = null;
    }
}

Deleting Category works fine, the relationship is updated correctly. If I delete a Contact however, I'm getting a constraint violation:

Caused by: org.h2.jdbc.JdbcBatchUpdateException: Referential integrity constraint violation: "FK34B085DF487AF903: 
PUBLIC.CATEGORY_CONTACT FOREIGN KEY(CONTACTS_ID) REFERENCES PUBLIC.CONTACT(开发者_如何学PythonID)"; SQL statement:
delete from Contact where id=? [23003-149]

How can I ensure that deleting a Contact will not delete the Category but will only remove the relationship?

EDIT: Duh! The issue was I also had a User object which had references to both Contact and Category. I missed clearing that relationship. The following is the change to preRemove() method in Contact class:

@PreRemove
public void preRemove() {
    for (Category c : this.categories) {
        c.contacts.remove(this);
    }

    this.user.contacts.remove(this);
    for (Category c : this.user.categories) {
        c.contacts.remove(this);
    }
    //It's important to save the user 
    this.user.save();
}

An other solution is to manually remove elements from the list and save it, before deleting the main entity :

while( !contact.categories.isEmpty() ) {  
    contact.categories.remove(0);  
}  
contact.save();  
contact.delete();