Spring form 如何绑定对象属性？

在使用spring 的 form 表单的时候，不知道如何绑定一个对象的属性。

如下：

Teacher 类

@Entity
@Table(name = "la_TEACHER")
public class Teacher extends IdEntity {

private String name;


// 老师的课程
private Set<Lesson> lessons;

@OneToMany(mappedBy = "teacher", cascade = CascadeType.ALL)
public Set<Lesson> getLessons() {
    return lessons;
}

public void setLessons(Set<Lesson> lessons) {
    this.lessons = lessons;
}

}

Lesson 类

@Entity
@Table(name = "LA_LESSON")
public class Lesson extends IdEntity {

//课题
private String name;

//老师 
private Teacher teacher;


public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

@ManyToOne(targetEntity = Teacher.class)
@JoinColumn(name = "teacher",nullable = false)
public Teacher getTeacher() {
    return teacher;
}

public void setTeacher(Teacher teacher) {
    this.teacher = teacher;
}

}

在lesson 的表单中， 使用select 来选择课程的教师，如下

/form:select

总是返回到 400 错误页面去了，没有其他的错误属性。 请熟悉spring from 的朋友帮忙看看怎么解决！谢谢！

请参考
[url]http://www.iteye.com/blogs/subjects/kaitao-springmvc[/url]