Android房间一对多关系

Question

我正在尝试建立一对多关系，我已经在线查看了多篇文章和教程，但所有示例都显示一个 table 与另一个 table 具有一对多关系。

在我的要求中，我有两个 table 与另一个 table 具有一对多关系（查看下图）

学生 table 有来自 Class 和学校 table 的 FK。他们通过 Class - 学生或学校 - 学生来解释一对多关系的大多数示例。

我想做 [Class, 学校] - 学生。

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

Answer 1

你的issue/problem没有说清楚。但是，以下是一个工作示例，它演示了根据您的模式提取关系的两种方法。

注意使用 Class 作为 class 充满了潜在的问题，根本不推荐。但是，以下内容确实使用了 Class 并规避了一些问题，可能无法完全反映您的架构。

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;
    }
}

Class（一个不明智的名字选择）:-

@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;
    }
}

Student 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 - Class StudentAndSchoolAndClass -（不使用 @Relation）

class StudentAndSchoolAndClass {

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

POJO 方法 2 - Class StudentWithSchoolWithClass -（使用 @Relation 的）

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@Relations 的 PJO 具有通过 JOIN 定义的关系

@Database class 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 和各自的提取一些数据POJO class.

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
            );
        }
    }
}

注意在第一个学校和 Class 名字是 class 的成员，而在第二个学校和 Class在列表中。由于学生只有一个 school/class，因此无需遍历列表，因为第一个 element/item (0) 将是列表中唯一的 element/item。

结果

当上面是运行（第一次）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

Android房间一对多关系

Android room one to many relationship

database

android

entity-relationship

one-to-many

android-room