hibernate中一种可替代联合主键的设计模式

有如下设计：

存在这样的表结构  学生表、课程表和分数表。

我们可以这样设计，一个学生对应多门课程，一门课程对应多个学生，他们之间是多对多的关系。我们可以建立一个中间表来关联他们，而此时恰巧有一个分数表来帮我充当这2个表的中间表的关系。

我们来分析一下，分数和学生，课程之间是这样的对应关系。一个具体的分数是某个学生、某门课程下的分数，是通过唯一的学生id和唯一的课程id来标识的。

所以，我们可以在score表上设置联合主键（student_id 和course_id）,但是联合主键有其不必要的复杂性，我们完全可以通过以下方式，来方便他的设计。

我们在score表中设计一个主键id，在设计一个外键student_id 和course_id.关系图如下

建表语句：

CREATE TABLE `student` (
  `id` int(11) NOT NULL,
  `name` varchar(20) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 PACK_KEYS=0;

CREATE TABLE `course` (
  `id` int(11) NOT NULL,
  `name` varchar(20) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 PACK_KEYS=0;

CREATE TABLE `score` (
  `id` int(11) NOT NULL,
  `student_id` int(11) default NULL,
  `course_id` int(11) default NULL,
  `score` int(3) default NULL,
  KEY `student_id` (`student_id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `score_fk2` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`),
  CONSTRAINT `score_fk1` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 PACK_KEYS=0;

对应的类文件语句如下：

import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * Student entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "student", catalog = "hibernate")
public class Student implements java.io.Serializable {

	// Fields

	/**
	 *
	 */
	private static final long serialVersionUID = 1L;
	private Integer id;
	private String name;
	private Set<Score> scores = new HashSet<Score>(0);

	// Constructors

	/** default constructor */
	public Student() {
	}

	/** minimal constructor */
	public Student(Integer id) {
		this.id = id;
	}

	/** full constructor */
	public Student(Integer id, String name, Set<Score> scores) {
		this.id = id;
		this.name = name;
		this.scores = scores;
	}

	// Property accessors
	@Id
	@Column(name = "id", unique = true, nullable = false)
	public Integer getId() {
		return this.id;
	}

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

	@Column(name = "name", length = 20)
	public String getName() {
		return this.name;
	}

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

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "student")
	public Set<Score> getScores() {
		return this.scores;
	}

	public void setScores(Set<Score> scores) {
		this.scores = scores;
	}

}

import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * Course entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "course", catalog = "hibernate")
public class Course implements java.io.Serializable {

	// Fields

	/**
	 *
	 */
	private static final long serialVersionUID = 1L;
	private Integer id;
	private String name;
	private Set<Score> scores = new HashSet<Score>(0);

	// Constructors

	/** default constructor */
	public Course() {
	}

	/** minimal constructor */
	public Course(Integer id) {
		this.id = id;
	}

	/** full constructor */
	public Course(Integer id, String name, Set<Score> scores) {
		this.id = id;
		this.name = name;
		this.scores = scores;
	}

	// Property accessors
	@Id
	@Column(name = "id", unique = true, nullable = false)
	public Integer getId() {
		return this.id;
	}

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

	@Column(name = "name", length = 20)
	public String getName() {
		return this.name;
	}

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

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "course")
	public Set<Score> getScores() {
		return this.scores;
	}

	public void setScores(Set<Score> scores) {
		this.scores = scores;
	}

}

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.Column;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

/**
 * Score entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "score", catalog = "hibernate")
public class Score implements java.io.Serializable {

	// Fields

	/**
	 *
	 */
	private static final long serialVersionUID = 1L;
	private ScoreId id;
	private Course course;
	private Student student;

	// Constructors

	/** default constructor */
	public Score() {
	}

	/** minimal constructor */
	public Score(ScoreId id) {
		this.id = id;
	}

	/** full constructor */
	public Score(ScoreId id, Course course, Student student) {
		this.id = id;
		this.course = course;
		this.student = student;
	}

	// Property accessors
	@EmbeddedId
	@AttributeOverrides({ @AttributeOverride(name = "id", column = @Column(name = "id", nullable = false)), @AttributeOverride(name = "studentId", column = @Column(name = "student_id")), @AttributeOverride(name = "courseId", column = @Column(name = "course_id")), @AttributeOverride(name = "score", column = @Column(name = "score")) })
	public ScoreId getId() {
		return this.id;
	}

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

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "course_id", insertable = false, updatable = false)
	public Course getCourse() {
		return this.course;
	}

	public void setCourse(Course course) {
		this.course = course;
	}

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "student_id", insertable = false, updatable = false)
	public Student getStudent() {
		return this.student;
	}

	public void setStudent(Student student) {
		this.student = student;
	}

}