如何通过子 POJO 的属性过滤复合 ManyToMany POJO？

Question

我有两个这样的房间实体：

@Entity
public class Teacher implements Serializable {
    @PrimaryKey(autoGenerate = true)
    public int id;

    @ColumnInfo(name = "name")
    public String name;
}

@Entity
public class Course implements Serializable {
    @PrimaryKey(autoGenerate = true)
    public short id;

    @ColumnInfo(name = "name")
    public String name;
}

...和一个连接点 table 用于像这样的多对多关系：

@Entity(primaryKeys = {"teacher_id", "course_id"})
public class TeachersCourses implements Serializable {
    @ColumnInfo(name = "teacher_id")
    public int teacherId;

    @ColumnInfo(name = "course_id")
    public short courseId;

    @ColumnInfo(index = true, name = "course_order")
    public short courseOrder;
}

...和一些复合 class 以获得某种“复合 POJO”：

public class TeacherWithCourses implements Serializable {
    @Embedded public Teacher teacher;
    @Relation(
            parentColumn = "id",
            entity = Course.class,
            entityColumn = "id",
            associateBy = @Junction(
                    value = TeachersCourses.class,
                    parentColumn = "teacher_id",
                    entityColumn = "course_id"
            )
    )
    public List<Courses> courses;
}

...那么，我就有了这种“复合DAO”：

@Dao
public abstract class TeacherWithCoursesDao {
    [...]

    // XXX This one works as expected
    @Transaction
    @Query("SELECT * FROM teacher " +
           "WHERE id=:teacher_id"
    )
    public abstract LiveData<List<TeacherWithCourses>> getTeachersByTeacherId(int teacher_id);

    // XXX FIXME
    // This one succeeds at loading "parents", but each "parent"'s list of "children" is empty
    @Transaction
    @Query("SELECT * FROM teacher " +
            "INNER JOIN teacherscourses AS tc ON teacher.id = tc.teacher_id " +
            "INNER JOIN course AS c ON c.id = tc.course_id " +
            "WHERE tc.course_id = :course_id " +
            "ORDER BY teacher.id ASC, tc.course_order ASC"
    )
    public abstract LiveData<List<TeacherWithCourses>> getTeachersByCourseId(short course_id);
}

问题的重点是...

有效的 returns 列表如预期：每个 TeacherWithCourses 都有教师和 List 课程。第二个没有：生成的 TeacherWithCourses 对象正确加载了 Teacher 属性，但是 List<Courses> 属性有一个空列表，尽管复杂的 SELECT查询基于预期的 INNER JOINS 个过滤器。

那么，我怎样才能像第一个 DAO 方法一样获得完整的 TeacherWithCourses 对象列表，而不是按课程 ID 过滤？

Answer 1

我认为您的问题是由于 列名称重复 并且基本上房间选择了不正确的值（我相信它使用最后一个，所以它会使用课程 ID教师 ID 的列值）。

即查询（使用 JOINS）将包含列：-

id（老师），
姓名（老师），
teacher_id,
course_id,
id（课程），
姓名（课程）

假设您在数据库中有以下内容：-

并使用了以下内容（LiveData 不习惯简洁方便）：-

    for(Course c: dao.getAllCourses()) {
        for (TeacherWithCourses tbc: dao.getTeachersByCourseId(c.id)) {
            Log.d("TEACHER","Teacher is " + tbc.teacher.name + " Courses = " + tbc.courses.size());
            for(Course course: tbc.courses) {
                Log.d("COURSE","\tCourse is " + course.name);
            }
        }
    }

那么结果就如你所报告的那样:-

2021-11-10 15:25:30.994 D/TEACHER: Teacher is Course1 Courses = 0
2021-11-10 15:25:30.996 D/TEACHER: Teacher is Course2 Courses = 0
2021-11-10 15:25:30.999 D/TEACHER: Teacher is Course3 Courses = 0
2021-11-10 15:25:30.999 D/TEACHER: Teacher is Course3 Courses = 0

但是（修复）

如果您使用不同的列名称，例如:-

@Entity
public class AltCourse implements Serializable {
    @PrimaryKey(autoGenerate = true)
    public short courseid; //<<<<<<<<<<

    @ColumnInfo(name = "coursename") //<<<<<<<<<<
    public String coursename; //<<<<<<<<<< doesn't matter

}

并且添加的数据基本上复制了原始课程（相同的 id #'s），所以 :-

连同：-

public class AltTeacherWithCourses implements Serializable {
    @Embedded
    public Teacher teacher;
    @Relation(
            parentColumn = "id",
            entity = AltCourse.class, //<<<<<<<<<< just to use alternative class
            entityColumn = "courseid", //<<<<<<<<<<
            associateBy = @Junction(
                    value = TeachersCourses.class,
                    parentColumn = "teacher_id",
                    entityColumn = "course_id"
            )
    )
    public List<AltCourse> courses; //<<<<<<<<<< just to use alternative class
}

注意到 teachercourses table 仅用于链接替代课程（而不是创建 altteachercourses table）

和:-

@Transaction
@Query("SELECT * FROM teacher " +
        "INNER JOIN teacherscourses AS tc ON teacher.id = tc.teacher_id " +
        "INNER JOIN altcourse AS c ON c.courseid = tc.course_id " +
        "WHERE tc.course_id = :course_id " +
        "ORDER BY teacher.id ASC, tc.course_order ASC"
)
public abstract List<AltTeacherWithCourses> getAltTeachersByCourseId(short course_id);

然后 :-

    for(Course c: dao.getAllCourses()) {
        for (AltTeacherWithCourses tbc: dao.getAltTeachersByCourseId(c.id)) {
            Log.d("TEACHER","Teacher is " + tbc.teacher.name + " Courses = " + tbc.courses.size());
            for(AltCourse course: tbc.courses) {
                Log.d("COURSE","\tCourse is " + course.coursename);
            }
        }
    }

即而不是 Course，AltCourse 被用在一个其他相同的地方，那么结果是：-

2021-11-10 15:41:09.223 D/TEACHER: Teacher is Teacher1 Courses = 3
2021-11-10 15:41:09.223 D/COURSE:   Course is AltCourse1
2021-11-10 15:41:09.223 D/COURSE:   Course is AltCourse2
2021-11-10 15:41:09.223 D/COURSE:   Course is AltCourse3
2021-11-10 15:41:09.225 D/TEACHER: Teacher is Teacher1 Courses = 3
2021-11-10 15:41:09.225 D/COURSE:   Course is AltCourse1
2021-11-10 15:41:09.225 D/COURSE:   Course is AltCourse2
2021-11-10 15:41:09.225 D/COURSE:   Course is AltCourse3
2021-11-10 15:41:09.229 D/TEACHER: Teacher is Teacher1 Courses = 3
2021-11-10 15:41:09.229 D/COURSE:   Course is AltCourse1
2021-11-10 15:41:09.229 D/COURSE:   Course is AltCourse2
2021-11-10 15:41:09.229 D/COURSE:   Course is AltCourse3
2021-11-10 15:41:09.230 D/TEACHER: Teacher is Teacher2 Courses = 1
2021-11-10 15:41:09.230 D/COURSE:   Course is AltCourse3

因此解决方案是

使用唯一的列名，或者
使用@Prefix 注解（@Embedded 的参数），例如你可以

:-

public class TeacherWithCourses implements Serializable {
    @Embedded(prefix = "prefix_teacher_") //<<<<<<<<<<
    public Teacher teacher;
    @Relation(
            parentColumn = "prefix_teacher_id", //<<<<<<<<<<
            entity = Course.class,
            entityColumn = "id",
            associateBy = @Junction(
                    value = TeachersCourses.class,
                    parentColumn = "teacher_id",
                    entityColumn = "course_id"
            )
    )
    public List<Course> courses;
}

并使用：-

@Transaction
@Query("SELECT teacher.id AS prefix_teacher_id, teacher.name AS prefix_teacher_name, c.* FROM teacher " +
        "INNER JOIN teacherscourses AS tc ON teacher.id = tc.teacher_id " +
        "INNER JOIN course AS c ON c.id = tc.course_id " +
        "WHERE tc.course_id = :course_id " +
        "ORDER BY teacher.id ASC, tc.course_order ASC"
)
public abstract List<TeacherWithCourses> getTeachersByCourseId(short course_id);

BUT 然后你还需要使用 :-

@Transaction
@Query("SELECT id AS prefix_teacher_id, name as prefix_teacher_name FROM teacher " +
        "WHERE id=:teacher_id"
)
public abstract List<TeacherWithCourses> getTeachersByTeacherId(int teacher_id);

补充评论：-

the only issue is that "ORDER BY" statement didn't seem to affect this "child list" 's sorting. But that one might be subject for a new question.

这个问题是由于@Relationship 的工作方式造成的。

@Relationship 通过底层查询获取 parent(s) 的 ALL @Relation objects 来工作。检索 children 时不考虑 @Query 中任何不影响检索到的 parents 的内容。因此，您无法控制订单。

也许可以考虑使用 CourseWithTeachers 方法，但这样您就无法控制教师的顺序。另一种方法是对 parent 和 child 使用 @Embedded，但你必须处理结果，即笛卡尔积，即每个 parent/child 组合的结果。

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