Android Room 一对多关系

Question

我正在尝试建立一对多关系，我在线检查了各种文章和教程，但所有示例都显示一个表与另一个表具有一对多关系。

在我的要求中，我有两个表与另一个表具有一对多关系（检查下图）

学生表有来自班级和学校表的 FK。大多数示例通过班级 - 学生或学校 - 学生解释一对多关系。

我想做[班级，学校] - 学生。

Core Data 在 iOS 上处理这个问题，但我无法在 Android 上解决这个问题。我什至尝试了 Android 文档中显示的嵌套关系示例，但这不起作用。

Answer 1

您的问题尚未明确说明。但是，以下是一个工作示例，演示了根据架构提取关系的两种方法。

• 请注意，使用 Class 作为类充满了潜在的问题，因此根本不建议这样做。但是，以下内容确实使用了该类并规避了一些可能无法完全反映您的架构的问题。

示例代码

学校实体：-

@Entity(tableName = "_school")
class School {
    @PrimaryKey
    @ColumnInfo(name = "school_id")
    Long Schoolid;
    @NonNull
    @ColumnInfo(name = "school_name")
    String SchoolName;

    School(){}

    @Ignore
    School(String schoolName) {
        this.SchoolName = schoolName;
    }
}

班级（名称的不明智选择）：-

@Entity(tableName = "_class")
class Class {
    @PrimaryKey
    @ColumnInfo(name = "class_id")
    Long ClassId;
    @NonNull
    @ColumnInfo(name = "class_name")
    String ClassName;

    Class(){}

    @Ignore
    Class(String className) {
        this.ClassName = className;
    }
}

学生实体（包括外键约束）：-

@Entity(
        tableName = "_student", foreignKeys = {
        @ForeignKey(
                entity = School.class,
                parentColumns = {"school_id"},
                childColumns = {"school_id"},
                onDelete = ForeignKey.CASCADE,
                onUpdate = ForeignKey.CASCADE
        ),
        @ForeignKey(
                entity = Class.class,
                parentColumns = {"class_id"},
                childColumns = {"class_id"},
                onDelete = ForeignKey.CASCADE,
                onUpdate = ForeignKey.CASCADE
        )
        }
)
class Student {
    @PrimaryKey
    @ColumnInfo(name = "student_id")
    Long StudentId;
    @ColumnInfo(name = "Student_name")
    String StudentName;
    @ColumnInfo(name = "school_id", index = true)
    Long SchoolId;
    @ColumnInfo(name = "class_id", index = true)
    Long ClassId;

    Student(){}

    @Ignore
    Student(String studentName, long schoolId, long classId) {
        this.StudentName = studentName;
        this.SchoolId = schoolId;
        this.ClassId = classId;
    }
}

POJO 方法 1 - 类StudentAndSchoolAndClass -（不使用@Relation）

class StudentAndSchoolAndClass {

    @Embedded
    Student student;
    String school_name;
    String class_name;
}

POJO 方法 2 - 类StudentWithSchoolWithClass - （使用@Relation's）

class StudentWithSchoolWithClass {

    @Embedded
    Student student;

    @Relation(entity = School.class,parentColumn = "school_id", entityColumn = "school_id")
    List<School> schoolList;
    @Relation(entity = Class.class,parentColumn = "class_id",entityColumn = "class_id")
    List<Class> classList;
}

Dao 接口AllDao

@Dao
interface AllDao {

    @Insert
    Long insertSchool(School s);
    @Insert
    Long insertClass(Class c);
    @Insert
    Long insertStudent(Student s);
    @Query("SELECT * FROM _school")
    List<School> getAllSchools();
    @Query("SELECT * FROM _school WHERE school_id = :school_id ")
    School getSchoolById(Long school_id);
    @Query("SELECT * FROM _class")
    List<Class> getAllClasses();
    @Query("SELECT * FROM _class WHERE class_id = :class_id")
    Class getClassById(Long class_id);
    @Query("SELECT * FROM _student JOIN _school ON _school.school_id = _student.school_id JOIN _class ON _class.class_id = _student.class_id")
    List<StudentAndSchoolAndClass> getStudentAndSchoolAndClass();
    @Query("SELECT * FROM _student")
    List<StudentWithSchoolWithClass> getStudentWithSchoolWithClass();

}

• 注意最后两个查询使用各自的 POJO，尤其是
• 带有 @Relations 的 PJO 具有通过 JOIN 定义的关系

@Database 类MyDatabase

@Database(entities = {School.class,Class.class,Student.class},version = 1)
abstract class MyDatabase extends RoomDatabase {
    abstract AllDao allDao();
}

最后一个 Activity MainActivity将一些数据加载到数据库中，然后使用 2 个 @Queries 和相应的 POJO 类提取一些数据。

public class MainActivity extends AppCompatActivity {

    MyDatabase db;
    AllDao allDao;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Build the MyDatabase instance
        db = Room.databaseBuilder(this,MyDatabase.class,"mydb")
                .allowMainThreadQueries()
                .build();
        // Build the allDao instance
        allDao = db.allDao();

        // Create some school objects
        School[] s_array = {new School("School1"),
                new School("School2"),
                new School("School3")
        };
        // Insert the Schools into the database
        for (School s: s_array) {
            allDao.insertSchool(s);
        }

        // Create some Class objects
        Class[] c_array = {
                new Class("Class1"),
                new Class("Class2"),
                new Class("Class3")
        };
        // Insert the classes
        for (Class c: c_array) {
            allDao.insertClass(c);
        }

        // Create some Student Objects
        Student[] st_array = {
                new Student("Fred",3,3), new Student("Mary",1,2)
        };
        //Insert the Students
        for(Student st: st_array) {
            allDao.insertStudent(st);
        }

        // Get the Students with the School and Class information using POJO 1 (realtionship via joins)
        List<StudentAndSchoolAndClass> sasac = allDao.getStudentAndSchoolAndClass();
        // Log the data
        for(StudentAndSchoolAndClass ssc: sasac) {
            Log.d("STUDENTINFO1","Student Name = " + ssc.student.StudentName +
                    "\n\t ID=" + ssc.student.StudentId + " SchoolID=" + ssc.student.SchoolId + " ClassID=" + ssc.student.ClassId +
                    "\n\t\t School Name = " + ssc.school_name +
                    "\n\t\t Class Name = " + ssc.class_name
                    );
        }
        // Get the Students with the School and Class information using POJO 2 (with @Relation's)
        List<StudentWithSchoolWithClass> swswc = allDao.getStudentWithSchoolWithClass();
        for(StudentWithSchoolWithClass ssc: swswc) {
            Log.d("STUDENTINFO2","Student Name = " + ssc.student.StudentName +
                    "\n\t ID=" + ssc.student.StudentId + " SchoolID=" + ssc.student.SchoolId + " ClassID=" + ssc.student.ClassId +
                    "\n\t\t School Name = " + ssc.schoolList.get(0).SchoolName +
                    "\n\t\t Class Name = " + ssc.classList.get(0).ClassName
            );
        }
    }
}

• 请注意，第一个学校和班级名称是班级的成员，而第二个学校和班级名称位于列表中。由于学生只有一个学校/班级，因此无需遍历列表，因为第一个元素/项目 (0) 将是列表中唯一的元素/项目。

结果

当以上运行（第一次）时，数据库是：-

日志包含：-

2021-04-01 22:09:51.977 D/STUDENTINFO1: Student Name = Fred
         ID=1 SchoolID=3 ClassID=3
             School Name = School3
             Class Name = Class3
2021-04-01 22:09:51.977 D/STUDENTINFO1: Student Name = Mary
         ID=2 SchoolID=1 ClassID=2
             School Name = School1
             Class Name = Class2


2021-04-01 22:09:51.982 D/STUDENTINFO2: Student Name = Fred
         ID=1 SchoolID=3 ClassID=3
             School Name = School3
             Class Name = Class3
2021-04-01 22:09:51.982 D/STUDENTINFO2: Student Name = Mary
         ID=2 SchoolID=1 ClassID=2
             School Name = School1
             Class Name = Class2