Android Room @Relation 多对多？

Question

我正在开发一个安卓应用程序，并在安卓操作系统中使用新的架构组件:LiveData、ViewModel 和 Room。
我对 Room 实现有一个小问题，即创建一个 @Relation，它从 JOIN 查询(​​多对多关系)返回结果。

我的数据库结构如下所示:

@Entity
public class Student{
  @PrimaryKey
  private int id;
  private String name;
  private String email;
} 

@Entity
public class Group{
  @PrimaryKey
  private int id;
  private String name;
}

@Entity(foreignKeys = {
            @ForeignKey(entity = Student.class,
                    parentColumns = "id",
                    childColumns = "student_id"),
            @ForeignKey(entity = Group.class,
                    parentColumns = "id",
                    childColumns = "group_id")
    })
public class StudentGroup{

  private int studentId;
  private int groupId;
}

我如何才能获得仅针对特定学生的所有组，像这样？

public class StudentWithGroups{
  @Relation(parentColumn = "id", entityColumn = "rule_id", entity = 
StudentGroup.class)
  private List<Group> groups;
}

我已经检查了 How can I represent a many to many relation with Android Room? 之类的问题和 Android Persistence room: "Cannot figure out how to read this field from a cursor"

Answer 1

带介绍Junction在房间里你可以轻松处理多对多的关系。

修改学生表和组表的主键为:

@Entity
public class Student{
  @PrimaryKey
  private int sId;
  private String name;
  private String email;
} 

@Entity
public class Group{
  @PrimaryKey
  private int gId;
  private String name;
}

@Entity(foreignKeys = {
        @ForeignKey(entity = Student.class,
                parentColumns = "sId",
                childColumns = "studentId"),
        @ForeignKey(entity = Group.class,
                parentColumns = "gId",
                childColumns = "groupId")
})
public class StudentGroup{
  private int studentId;
  private int groupId;
}

您可以获得所有特定学生组:

 public class StudentWithGroups{
   @Embedded
   Student student;
   @Relation(
         parentColumn = "sId",
         entity = Group.class,
         entityColumn = "gId",
         associateBy = @Junction(
                 value = StudentGroup.class,
                 parentColumn = "studentId",
                 entityColumn = "groupId)
   )
   List<Group> groups;
 }

现在您可以查询数据库以获取结果:

 @Dao
 public interface StudentDao {
   @Query("SELECT * FROM Student")
   List<StudentWithGroups> getGroupsOfStudent();
}