Android 如何嵌入非'；没有直接关系吗？

我有三个基本实体来表示我房间数据库中的表：国家、州和城市

为了在一个州内嵌入城市列表，我创建了一个POJO，名为StateWithCities：

package org.example.roomtest;

import java.util.List;
import androidx.room.Embedded;
import androidx.room.Relation;

public class StateWithCities {

    @Embedded
    public State state;

    @Relation(
            parentColumn = "_id", // This is the `states`.`_id` column.
            entityColumn = "state_id" // This is the `cities`.`state_id` column.
    )
    public List<City> cities;
}

@Query("SELECT * FROM `countries`")
LiveData<List<CountryWithStatesWithCities>> getAllCountriesWithStatesWithCities();

在这一点上，一切都很好。但当我将此查询添加到CountryDao时：

package org.example.roomtest;

import java.util.List;
import androidx.room.Embedded;
import androidx.room.Relation;

public class StateWithCities {

    @Embedded
    public State state;

    @Relation(
            parentColumn = "_id", // This is the `states`.`_id` column.
            entityColumn = "state_id" // This is the `cities`.`state_id` column.
    )
    public List<City> cities;
}

@Query("SELECT * FROM `countries`")
LiveData<List<CountryWithStatesWithCities>> getAllCountriesWithStatesWithCities();

及

…CountryWithStatesWithCities.java:16:错误：在org.team\u love.shapethefuture.roomtest.StateWithCities中找不到子实体列'country\u id'。选项：
>公共城市名单；
>                                  ^

---

这些错误似乎告诉我，我的CountryDao接口需要引用实体（或数据库视图），而不是POJO。那么，解决我的需求的正确方法是什么：如何嵌入不直接相关的文件室实体？

您需要设置实体类来明确描述

CountryWithStatesWithCities

，因为文件室在从上下文推断时存在问题：

public class CountryWithStatesWithCities {

    @Embedded
    public Country country;

    @Relation(
            entity = State.class, <- this should be added explicitly
            parentColumn = "_id", // This is the `country`.`_id` column.
            entityColumn = "country_id" // This is the `states`.`country_id` column.
    )
    public List<StateWithCities> stateWithCities;
}

公共类国家和州/市{
@嵌入
公共国家；
@关系(
实体=State.class，
> ...CountryWithStatesWithCities.java:16: error: Cannot find the child entity column `country_id` in org.team_love.shapethefuture.roomtest.StateWithCities. Options: 
>     public List<StateWithCities> stateWithCities;
>                                  ^

public class CountryWithStatesWithCities {

    @Embedded
    public Country country;

    @Relation(
            entity = State.class, <- this should be added explicitly
            parentColumn = "_id", // This is the `country`.`_id` column.
            entityColumn = "country_id" // This is the `states`.`country_id` column.
    )
    public List<StateWithCities> stateWithCities;
}