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:

@Entity
public class Score {
    @Id
    private String id;
    private int score;
}

@Entity
public 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<Student> 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:


@Entity
public class Student {
    @Id
    private String id;
    private String name;
    private int age;
    @ToMany(referencedJoinProperty = "studentId")
    private List<Score> scores;
}

@Entity
public 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<Student> 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ToManyJoinEntity通过JoinEntity注解来配置关联的建。如下:

@Entity
public class Student {
    @Id
    private String id;
    private String name;
    private int age;

    @ToMany
    @JoinEntity(
            entity = Join.class,
            sourceProperty = "studentId",
            targetProperty = "scoreId"
    )
    private List<Score> scores;
}
@Entity
public class Join {
    @Id
    private String id;
    private String studentId;
    private String scoreId;
}
@Entity
public 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getset方法。这个是时候就要注意了。来看下生成的代码:

@Entity
public class Point {
    @Id
    private Long id;
    private Long strokeId;
    private int x;
    private int y;
}

@Entity
public class Stroke {
    @Id
    private Long id;
    private String name;
    @ToMany(referencedJoinProperty = "strokeId")
    private List<Point> 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getset方法。
我们看下Stroke的一个get方法我们会看到下面这些代码。就由于这个代码。可能就会导致。我们解析到了Stroke后调用getPoints()方法想要获取点的集合是出现问题,这时候就可能会报错。这个时候我们可以在单独写另外的一个get方法,来支持直接获取points对象。

 @Generated(hash = 404164872)
    public List<Point> 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<Point> pointsNew = targetDao._queryStroke_Points(id);
            synchronized (this) {
                if(points == null) {
                    points = pointsNew;
                }
            }
        }
        return points;
    }
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