在Android Room中进行多对多查询可以通过嵌套项ID进行选择。以下是一个示例解决方法:
首先,你需要定义一个数据模型类来表示多对多关系,例如一个学生和一个课程之间的关系:
@Entity(tableName = "students")
data class Student(
@PrimaryKey val studentId: Int,
val studentName: String
)
@Entity(tableName = "courses")
data class Course(
@PrimaryKey val courseId: Int,
val courseName: String
)
@Entity(tableName = "student_course",
primaryKeys = ["studentId", "courseId"],
foreignKeys = [
ForeignKey(entity = Student::class, parentColumns = ["studentId"], childColumns = ["studentId"]),
ForeignKey(entity = Course::class, parentColumns = ["courseId"], childColumns = ["courseId"])
]
)
data class StudentCourse(
val studentId: Int,
val courseId: Int
)
然后,创建一个Dao接口来定义多对多查询的方法:
@Dao
interface StudentCourseDao {
@Transaction
@Query("SELECT * FROM students WHERE studentId IN (SELECT studentId FROM student_course WHERE courseId = :courseId)")
fun getStudentsForCourse(courseId: Int): List
@Transaction
@Query("SELECT * FROM courses WHERE courseId IN (SELECT courseId FROM student_course WHERE studentId = :studentId)")
fun getCoursesForStudent(studentId: Int): List
}
在这个示例中,我们使用了嵌套查询来选择具有特定courseId的学生,以及具有特定studentId的课程。
最后,在你的数据库类中,添加一个抽象方法来获取StudentCourseDao实例:
@Database(entities = [Student::class, Course::class, StudentCourse::class], version = 1)
abstract class MyDatabase : RoomDatabase() {
abstract fun studentCourseDao(): StudentCourseDao
}
现在,你可以在你的代码中使用StudentCourseDao来执行多对多查询:
val database = Room.databaseBuilder(applicationContext, MyDatabase::class.java, "my-database").build()
val studentCourseDao = database.studentCourseDao()
val studentsForCourse = studentCourseDao.getStudentsForCourse(courseId)
val coursesForStudent = studentCourseDao.getCoursesForStudent(studentId)
这样,你就可以根据嵌套项ID选择多对多关系。