JPA EntityManager question

I'm quite new to both JPA/Hibernate and to Java, and am having some trouble with the basics of using the EntityManager class to manage persistent objects. I would appreciate it very much if someone would explain something very basic to me, because I can't figure it out from the documentation.

JPA 2 / Hibernate / Postgresql in a JSE environment.

* Class definitions below *

The following works as I would expect it to:

em.getTransaction().begin();
Car corolla = new Car();
Part clutch = new Part();
clutch.setCar( corolla );
Part bumper = new Part();
bumper.setCar( corolla );
em.persist( corolla );
em.persist( clutch );
em.persist( bumper );
em.getTransAction().commit();

But this does not remove the links from the parts to the cars in the database:

tx.begin();
corolla.getParts().clear();
tx.commit();

Why is this the case?

Thanks in advance, and sorry if this is a silly question.

Mike.

Car class:

@Entity
public class Car {

private Long id;
private Set<Part> parts;

....

public Car() { parts = new HashSet<Part>(); }

@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }

@OneToMany( mappedBy="car", cascade=CascadeType.ALL )
public Set<Part> getParts() { return this.parts; }
public void setParts( Set<Part> parts ) { this.parts = parts; }

....

}

Part class:

@Entity
public class Part {

private Long id;
private Car car;

...

public Part() {};

@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }

@ManyToOne
@JoinColumn( name="car_id" )
public Car getCar() { return thi开发者_如何学JAVAs.car; }
public void setCar( Car car ) { this.car = car; }

...

}

When sides of bidirectional relationship disagree, a state of the owning side (i.e. a side without mappedBy) is persisted to the database. So, you need to modify both sides consitently:

for (Part p: corolla.getParts()) {
    p.setCar(null);
}
corolla.getParts().clear();

It looks fine, however while removing an entity from your database you should be using:

EntityManager.remove