之前我们看到了greenDao的简单使用，但是就这些是远远不够的，有时候我们需要存储的数据较为复杂，这个时候我们可能需要使用到多表关联的操作。

ToOne

一对一的关系映射。看个例子：

@Entitypublic class Score {    @Id    private String id;    private int score;}@Entitypublic class Student {    @Id    private String id;    private String name;    private int age;    private String scoreId;    @ToOne(joinProperty = "scoreId")    private Score score;}        //先向数据库中插入两条数据        Score score = new Score("1101", 80);        Student magicer = new Student("110","Magicer",12,"1101");        scoreDao.insertOrReplace(score);        studentDao.insertOrReplace(magicer);                //之后查找我们插入的数据，就可以查询出来我们想要的带有成绩的学生实体。        QueryBuilder
    
      queryBuilder = studentDao.queryBuilder().where(StudentDao.Properties.Name.eq("Magicer"));        for (Student student : queryBuilder.list()) {            Log.i(TAG, "onCreate: "+student.toString());        }
    

在上面的例子中，我们设定每个学生有一门成绩，这个时候就是个ToOne一对一的关系。我们通过joinProperty来设置外键。我们就可以很方便的查询出某个学生的成绩了。

public @interface ToOne {    /**     * Name of the property inside the current entity which holds the key of related entity.     * If this parameter is absent(缺少的), then an additional column is automatically created to hold the key.     */    String joinProperty() default "";}

ToMany

但是一般一个学生会有多个成绩，这个时候我们就需要使用ToMany一对多的关系了。先看下例子：

@Entitypublic class Student {    @Id    private String id;    private String name;    private int age;    @ToMany(referencedJoinProperty = "studentId")    private List
    
      scores;}@Entitypublic class Score {    @Id    private String id;    private int score;    private String type;    private String studentId;}        Score math = new Score("1101", 87, "Math", "110");        Score english = new Score("1102", 99, "English", "110");        Score chinese = new Score("1103", 120, "Chinese", "110");        scoreDao.insertOrReplaceInTx(math,english,chinese);//使用事务插入或替换数据        Student magicer = new Student("110", "Magicer", 23);        studentDao.insertOrReplace(magicer);        Query
     
       query = studentDao.queryBuilder().where(StudentDao.Properties.Name.eq("Magicer")).build();        for (Student student : query.list()) {            Log.i(TAG, "onCreate: "+student);        }//I/MainActivity: onCreate: Student{id='110', name='Magicer', age=23, score=[Score{id='1101', score=87, type='Math', studentId='110'}, Score{id='1102', score=99, type='English', studentId='110'}, Score{id='1103', score=120, type='Chinese', studentId='110'}]}
     
    

这个时候，一个学生就有Math Enghlish Chinese三个的成绩。这个时候，我们使用referencedJoinProperty 将成绩跟学生建立了关联关系。

public @interface ToMany {    /**     * Name of the property inside the target entity which holds id of the source (current) entity     * Required unless no {@link JoinProperty} or {@link JoinEntity} is specified     */    String referencedJoinProperty() default "";    /**     * Array of matching source -> target properties     * Required unless {@link #referencedJoinProperty()} or {@link JoinEntity} is specified     */    JoinProperty[] joinProperties() default {};}

JoinEntity

有时我们还要创建多对多的关联关系N:M。在greenDao中就使用JoinEntity注解；先来看下他的定义：

public @interface JoinEntity {    /** Reference to join-entity class, which holds the source and the target properties */    Class
     entity();    /** Name of the property inside the join entity which holds id of the source (current) entity */    String sourceProperty();    /** Name of the property inside the join entity which holds id of the target entity */    String targetProperty();}

配置多对多关系的时候我们需要使用到ToMany和JoinEntity通过JoinEntity注解来配置关联的建。如下：

@Entitypublic class Student {    @Id    private String id;    private String name;    private int age;    @ToMany    @JoinEntity(            entity = Join.class,            sourceProperty = "studentId",            targetProperty = "scoreId"    )    private List
    
      scores;}@Entitypublic class Join {    @Id    private String id;    private String studentId;    private String scoreId;}@Entitypublic class Score {    @Id    private String id;    private int score;    private String type;    private String studentId;}
    

遇到的问题

当插入到数据库中的数据是网络请求得到的时候会有些注意事项。由于greenDao会帮助我们生成一些get和set方法。这个是时候就要注意了。来看下生成的代码：

@Entitypublic class Point {    @Id    private Long id;    private Long strokeId;    private int x;    private int y;}@Entitypublic class Stroke {    @Id    private Long id;    private String name;    @ToMany(referencedJoinProperty = "strokeId")    private List
    
      points;}
    

如上面，我们现在有每个笔画Stroke会有很多的Point。编译下之后会生成很多get和set方法。

@Generated(hash = 404164872)    public List
    
      getPoints() {        if (points == null) {            final DaoSession daoSession = this.daoSession;            if (daoSession == null) {                throw new DaoException("Entity is detached from DAO context");            }            PointDao targetDao = daoSession.getPointDao();            List
     
       pointsNew = targetDao._queryStroke_Points(id);            synchronized (this) {                if(points == null) {                    points = pointsNew;                }            }        }        return points;    }