Hibernate updates @Version property of entity when adding items to @OneToMany collection - is this normal?

I have two Foo and Bar entities which have a 1:n relationship:

@Entity
public class Foo {
    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "foo_id")
    Integer id;

    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name = "foo_id")
    private Set<Bar> bars;

    @NotNull
    @Version
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="update_date", nullable=false)
    private Date updateDate;

    // getters and setters ...
}

@Entity
public class Bar{
    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "bar_id")
    Integer id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "foo_id")
    private Foo foo;


    // getters and setters ...
}

Now I add a Bar to an 开发者_运维技巧existing Foo:

Foo foo = entityManager.find(Foo.class, 1);
Bar bar = new Bar();
entityManager.persist(bar);
foo.getBars().add(bar);
entityManager.flush();

The strange thing is that foo.updateDate gets updated when I call entityManager.flush(), even though the only thing that should happen in the database is an INSERT of a new row to Bar - i.e. the existing Foo row should not be modified. Is this behavior of the Hibernate EntityManager normal / intentional?