Android Room JOIN查询将结果分割成大小为1的列表。

Question 1

我有2个安卓室表book和persons。每本书都有一个外键，叫做owner_id，它把它分配给一个人的实体。我现在试图得到一个地图，给我一个书与每个人的列表。

为此，我在我的DAO类中使用这个方法。

@Transaction
@Query("SELECT * FROM books JOIN persons ON persons.id = book.owner_id")
public abstract Map<Person, List<Book>> getMap();
这将返回一个像这样的地图。
而不是我的理想结果。
我的房间班级是这样的。
@Entity
public class Person {
    @NonNull
    @PrimaryKey
    public UUID id;
    public String name="";
    @Override
    public boolean equals(@Nullable Object obj) {
        if(obj instanceof  Person)
            return ((Person) obj).equals(id);
        return super.equals(obj);
    @Override
    public int hashCode() {
        return id.hashCode();
@Entity
public class Book {
    @NonNull
    @PrimaryKey
    public UUID id;
    public UUID owner_id;
    public String name="";
    @Override
    public boolean equals(@Nullable Object obj) {
        if(obj instanceof  Book)
            return ((Book) obj).equals(id);
        return super.equals(obj);
    @Override
    public int hashCode() {
        return id.hashCode();
@Database(entities = {Person.class,Book.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class LibarayDB extends RoomDatabase {
    public abstract LibarayDBdao libarayDBdao();
我怎样才能使返回的地图只包含关键的Person X一次，并有一个列表的
书籍A和书籍B作为一个值？

Question 2


          
           
            你使用一个嵌入了
            
             Person
            
            成员的POJO，即注释了
            
             @Embedded
            
            和一个与List相关的，即注释了
            
             @Relation
            
            。
           
           
            You then use an
            
             @Query
            
            , preceded by
            
             @Transaction
            
            , to
            
             SELECT
            
            the respective
            
             Person
            
            s (no need for the
            
             JOIN
            
            ) returning a List<> of the POJO.
           
           
            作为一个例子:-
           
           class PersonWithListOfRelatedBooks {
   @Embedded
   Person person;
   @Relation(
           entity = Book.class,
           parentColumn = "id",
           entityColumn = "owner_id"
   List<Book> bookList;
@Transaction
@Query("SELECT * FROM person")
List<PersonWithListOfRelatedBooks> getPersonWithRelatedBooks();
Using code modified for convenience
Longs for id's so no need for type converters
.allowMainThreadQueries, so can run on the main thread
The Book类:-
@Entity(
        /* ADDED as a suggestion (enforces referential integrity) */
        /* NOTE will fail if child(Book) is added without a parent (Person) */
        foreignKeys = {
                @ForeignKey(
                        entity = Person.class,
                        parentColumns = {"id"},
                        childColumns = {"owner_id"},
                        /* Optional but helps to maintain referential integrity */
                        onDelete = ForeignKey.CASCADE,
                        onUpdate = ForeignKey.CASCADE
public class Book {
    @NonNull
    @PrimaryKey
    public Long id; /* CHANGED FOR convenience */
    @ColumnInfo
    public Long owner_id; /* CHANGED FOR convenience */
    public String name="";
    @Override
    public boolean equals(@Nullable Object obj) {
        if(obj instanceof  Book)
            return ((Book) obj).equals(id);
        return super.equals(obj);
    @Override
    public int hashCode() {
        return id.hashCode();
The Person类:-
@Entity
public class Person {
   @NonNull
   @PrimaryKey
   public Long id;  /* CHANGED FOR convenience */
   public String name="";
   @Override
   public boolean equals(@Nullable Object obj) {
      if(obj instanceof  Person)
         return ((Person) obj).equals(id);
      return super.equals(obj);
   @Override
   public int hashCode() {
      return id.hashCode();
The 利巴雷道接口 :-
interface LibarayDBdao {
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    long insert(Person person);
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    long insert(Book book);
    @Transaction
    @Query("SELECT * FROM person")
    List<PersonWithListOfRelatedBooks> getPersonWithRelatedBooks();
The 有相关书籍的人的名单（PersonWithListOfRelatedBooks类（同上）。
The 利巴雷数据库 abstract类:-
@Database(entities = {Person.class,Book.class}, version = 1, exportSchema = false /* ADDED to suppress warning */)
//@TypeConverters({Converters.class}) /* Commented out for convenience */
public abstract class LibarayDB extends RoomDatabase {
    public abstract LibarayDBdao libarayDBdao();
    private static volatile LibarayDB instance;
    public static LibarayDB getInstance(Context context) {
        if (instance==null) {
            instance = Room.databaseBuilder(context,LibarayDB.class,"libaray.db")
                    .allowMainThreadQueries() /* for convenience and brevity of the demonstration */
                    .build();
        return instance;
一些活动代码：-
public class MainActivity extends AppCompatActivity {
    LibarayDB db;
    LibarayDBdao dao;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        db = LibarayDB.getInstance(this);
        dao = db.libarayDBdao();
        Person personX = new Person();
        personX.name = "PersonX";
        long personXId = dao.insert(personX);
        Book book = new Book();
        book.name = "BookA";
        book.owner_id = personXId;
        dao.insert(book);
        book.name = "BookB";
        dao.insert(book);
        StringBuilder sb = new StringBuilder();
        for(PersonWithListOfRelatedBooks pwrb: dao.getPersonWithRelatedBooks()) {
            for (Book b: pwrb.bookList) {
                sb.append("\n\tBook is " + b.name);
            Log.d("DBINFO","Person is " + pwrb.person.name + " and has " + pwrb.bookList.size() + " books:-" + sb);
RESULT(作为对日志的输出) :-
D/DBINFO: Person is PersonX and has 2 books:-
        Book is BookA
        Book is BookB