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我如何通過兒童POJO的屬性來合成ManyToMany POJO？

發布時間：2024-03-09 00:19:56 作者：網友整理

本文介紹了我如何通過兒童POJO的屬性來合成ManyToMany POJO？的處理方法，對大家解決問題具有一定的參考價值，需要的朋友們下面隨著小編來一起學習吧！

問題描述

我有兩個類似的Room實體：

@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;
}

.和多對多關系的連接表，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;
}

.和一些用于獲取某種類型的復合POJO&QOOT；的復合類：

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，我有這種復合刀(&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);
}

問題的重點是.

工作的那個會按預期返回列表：每個TeacherWithCourses都有老師和List課程。第二個并非如此：生成的TeacherWithCourses對象正確加載了Teacher屬性，但是List<Courses>屬性有一個空列表，盡管基于INNER JOINS的復雜SELECT查詢按預期篩選。

那么，如何像第一個DAO方法一樣獲取完整的TeacherWithCourses對象列表，但改為按課程ID進行篩選？

推薦答案

我認為您的問題是由于列名重復，并且基本上房間選擇了不正確的值(我認為它使用最后一個值，因此將使用課程ID列值作為教師ID)。

即查詢(帶有聯接)將由列組成：-

id(教師)，
姓名(教師)，
教師id，
Course_id，
id(課程)，
名稱(課程)

假設您的數據庫中有以下內容：-

并且使用了(LiveData不習慣簡潔方便)：-

    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，結果如您所報告的那樣：-

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

}

添加的數據基本上復制了原始課程(相同的ID#)，因此：-

與：-

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
}

請注意，使用教師課程表只是說明鏈接了替代課程(而不是創建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，當然不使用AltCourse，而是在其他相同的中使用AltCourse，則結果為：-

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，解決方案是

    使用唯一列名，或
    使用@Prefix注釋(@Embedded的參數)，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);

但是您還需要使用：-

@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);

其他評論：-

唯一的問題是&quot；ORDER BY&QOOT；語句似乎不會影響此&QOOT；子列表&QOOT；的排序。但該子列表可能會出現新問題。

該問題是由于@Relationship的工作方式造成的。

@Relationship通過基礎查詢獲取父級的所有@Relation對象。在檢索子查詢時，不會考慮@查詢中任何不影響所檢索的父級的內容。因此，您無法控制訂單。

也許可以考慮”與教師一起上課”的方法，但這樣您就無法控制教師的順序了。另一種方法是對父項和子項使用@Embedded，但隨后必須處理結果，即每個父項/子項組合的結果。

這篇關于我如何通過兒童POJO的屬性來合成ManyToMany POJO？的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，

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標簽：ManyToMany POJO 兒童 合成 屬性
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