我如何通过儿童POJO的属性来合成ManyToMany POJO？
我有两个类似的Room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;
}
.和多对多关系的连接表，如下所示：
@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;
}
.和一些用于获取某种类型的复合POJO&QOOT；的复合类：
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，我有这种复合刀(&Q；)：
@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.
工作的那个会按预期返回列表：每个TeacherWithCourses都有老师和List课程。第二个并非如此：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，尽管基于INNER JOINS的复杂SELECT查询按预期筛选。
那么，如何像第一个DAO方法一样获取完整的TeacherWithCourses对象列表，但改为按课程ID进行筛选？
解决方案
我认为您的问题是由于列名重复，并且基本上房间选择了不正确的值(我认为它使用最后一个值，因此将使用课程ID列值作为教师ID)。
即查询(带有联接)将由列组成：-
- id(教师)，
- 姓名(教师)，
- 教师id，
- Course_id，
- id(课程)，
- 名称(课程)
假设您的数据库中有以下内容：-
并且使用了(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"," Course is " + course.name);
}
}
}
然后，结果如您所报告的那样：-
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)
如果使用不同的列名，例如：-
@Entity
public class AltCourse implements Serializable {
@PrimaryKey(autoGenerate = true)
public short courseid; //<<<<<<<<<<
@ColumnInfo(name = "coursename") //<<<<<<<<<<
public String coursename; //<<<<<<<<<< doesn't matter
}
添加的数据基本上复制了原始课程(相同的ID#)，因此：-
与：-
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
}
- 请注意，使用教师课程表只是说明链接了替代课程(而不是创建altTeacherCourses表)
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"," Course is " + course.coursename);
}
}
}
即，当然不使用AltCourse，而是在其他相同的中使用AltCourse，则结果为：-
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
- 使用唯一列名，或
- 使用@Prefix注释(@Embedded的参数)，例如您可以使用
：-
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);
但是您还需要使用：-
@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);
其他评论：-
唯一的问题是"；ORDER BY&QOOT；语句似乎不会影响此&QOOT；子列表&QOOT；的排序。但该子列表可能会出现新问题。
该问题是由于@Relationship的工作方式造成的。
@Relationship通过基础查询获取父级的所有@Relation对象。在检索子查询时，不会考虑@查询中任何不影响所检索的父级的内容。因此，您无法控制订单。
也许可以考虑"与教师一起上课"的方法，但这样您就无法控制教师的顺序了。另一种方法是对父项和子项使用@Embedded，但随后必须处理结果，即每个父项/子项组合的结果。
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