Question

If a parent object starts off with a list of 3 children objects, removes child 1 and calls session.update(parent), what kind of hibernate mapping would I need in order to clear the parent PK from the removed child table without deleting the removed child completely?

I also have a constraint which prevents any parent from being deleted if any child is assign to it and another constraint that prevents me from accidentally cascade-deleting any child entries.

I can successfully cascade update children by simply calling session.update(parent), but I’m having trouble doing the reverse.

public class Parent implements Serializable {
  private static final long serialVersionUID = 1L;
  private Long id;
  private List<Child> children;

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  @OneToMany(mappedBy = "parent")
  @org.hibernate.annotations.Cascade( 
        value = {org.hibernate.annotations.CascadeType.SAVE_UPDATE})
  public List<Child> getChildren() {
    return children;
  }

  public void setChildren(List<Child> children){
    this.children = children;
    for(Child child : children) {
      child.setParent(this);
    }
  }

  //-----------------   Would I need something like this?  ---------------
  //----------------- Or does hibernate have a better way? ---------------
  public void setChildren(List<Child> children){

    for(Child child : this.children)
        if (!children.contains(child)) {
          child.setParent(null);
        }
    }
    this.children = children;
    for(Child child : children) {
      child.setParent(this);
    }
  }
  //----------------------------------------------------------------
}

public class Child implements Serializable {
  private static final long serialVersionUID = 1L;
  private Long id;
  private Parent parent;

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  @ManyToOne
  public Parent getParent() {
    return parent;
  }

  public void setParent(Parent parent) {
    this.parent = parent;
  }
}

This solution is similar, but I’m not deleting the Parent, instead I’m removing a Child from the List:

hibernate cascade – update child to null

Answer 1

If you don’t have a not-null constraint on the FK parent field in the child table, you can just set parent to null and save the child.

I’d write the following method:

Boolean removeChild(Child c) {
    Boolean success = getChildren().remove(c);
    if(success) c.setParent(null);
    return success;
 }

I’d be careful about overwriting the setChildren method, as hibernate needs plain getters and setters.