如何通過子 POJO 的屬性過濾複合多對多 POJO？

我相信您的問題是由於列名被重複並且基本上房間選擇了不正確的值（我相信它使用最後一個所以它將使用課程 ID 列值作為教師 ID）。

那就是查詢（帶有 JOINS）將由列組成：-

• 身份證（老師），
• 姓名（老師），
• 老師_id，
• 課程編號，
• 身份證（課程），
• 姓名（課程）

因此，假設您在數據庫中有以下內容：-

並使用以下內容（LiveData 不用於簡潔和方便）：-

    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);
            }
        }
    }

然後結果，就像你報告的那樣：-

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（修復）

如果您使用不同的列名，例如：-

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

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

}

• 並且添加的數據基本上複製了原始課程（相同的 id #'s），因此：-

• 

隨著 ：-

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
}

• noting that the teachercourses table is used just that the alternative Courses are linked (rather than create an altteachercourses table)

and :-

@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);

and then :-

    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);
            }
        }
    }

i.e. instead of Course, AltCourse is used in an otherwise an identical, then the result is :-

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

As such the solution is to either

1. use unique column names, or
2. use the @Prefix annotation (parameter of the @Embedded) e.g. you could have

:-

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;
}

and use :-

@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 you would then also need to use :-

@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);

Additional re comment :-

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.

The issue is due to how @Relationship works.

@Relationship通過獲取工作全部在@Relation通過基礎查詢父（S）的對象。檢索子項時不考慮@Query 中不影響檢索到的父項的任何內容。因此，您無法控制 ORDER。

也許考慮 CourseWithTeachers 方法，但是您無法控制教師的訂單。另一種方法是將@Embedded 用於父級和子級，但是您必須處理結果，即角叉乘積，即每個父/子組合的結果。