java - Android Room - 处理对象中的对象列表和查询结果

Question

我在使用 Android 的房间数据库，但在理解如何执行以下操作时遇到了一些大问题:

1. 当我添加 Person到数据库，它添加所有变量，例如List<Shoe> , List<Pet>等到数据库。

2. 创建关系，以便在我检索 Person 时，它的所有字段都被检索，例如 Pet, Shoe, Shirt等等(不确定是什么类型的查询)

3. 执行简单查询，例如Retrieve Person where鞋名= "boot" ;

我知道您必须对对象列表使用外键关系，否则对于单个对象可以使用 @Embed或 @TypeConverter

示例代码如下；

@Entity(tableName = "person")
    public class Person {
        @PrimaryKey
        @NonNull
        private String personId;
        private List<Pet> pets;
        private List<Shoe> shoes;
        private List<Shirt> shirts;
    }

@Entity(foreignKeys = {
        @ForeignKey(
                entity = Person.class,
                parentColumns = "personId",
                childColumns = "personIdFk"
        )
    })
    public class Pet {
        String petId;
        String name;
        String personIdFk; //Foreign key to person
    }

@Entity(foreignKeys = {
            @ForeignKey(
                    entity = Person.class,
                    parentColumns = "personId",
                    childColumns = "personIdFk"
            )
    })
    public class Shoe {
        String shoeId;
        String name;
        String personIdFk; //Foreign key to person
    }


@Entity(foreignKeys = {
        @ForeignKey(
                entity = Person.class,
                parentColumns = "personId",
                childColumns = "personIdFk"
        )
    })
    public class Shirt {
            String shirtId;
            String name;
            String personIdFk; //Foreign key to person
        }

Answer 1

由于某些 potential issues with lazy loading，Room 不直接支持此功能，但有一些 DAO 技巧是可能的。您需要显式处理插入，并立即查询所有内容，您需要一个 POJO 来包装所有内容。

@Entity(foreignKeys = {
    @ForeignKey(
            entity = PersonEntity.class,
            parentColumns = "personId",
            childColumns = "personIdFk",
            onDelete = CASCADE
    )
})
public class Pet {
    @PrimaryKey
    private String petId;
    private String name;
    private String personIdFk;
}

@Entity(tableName = "person")
public class PersonEntity {
    @PrimaryKey
    private String personId;
}

public class Person {
    @Embedded
    private PersonEntity personEntity;
    @Relation(parentColumn = "personId", entityColumn = "personIdFk")
    private List<Pet> pets;
}

@Dao
public abstract class PersonDao {

    @Insert
    protected abstract void insert(PersonEntity personEntity);

    @Insert
    protected abstract void insert(List<Pet> pets);

    @Transaction
    public void insert(Person person) {
        insert(person.getEntity());
        insert(person.getPets());
    }

    @Query("SELECT * FROM person")
    public abstract List<Person> getAll();

}

@Database(entities = {PersonEntity.class, Pet.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {

    public abstract PersonDao personDao();

}

为简洁起见省略了构造函数、getter 和 setter。