Android 房间持久性库

Question

我是 Room 的新手，@Relation 对我来说不是很清楚。如果我理解正确，我有实体，例如(RSS)ChannelEntity， channel 具有名为 ItemEntity 的项目。这些是带有@Entity 注释的类。我还有一个 POJO 来“连接”我的实体。我的意思是我必须像这样写一个 POJO:

public class Channel { 

   @Embedded
   private ChannelEntity channel;

// Link is the primary key in ChannelyEntity
   @Relation (parentColumn = "link", entityColumn = "channel_link")
   private ArrayList<ItemEntity> items;

// Getters and Setters are here
}

然后我必须编写一个 dao 接口(interface)，我可以像这样获取 Channel(不是 ChannelEntity):

public interface ChannelDao {

    @Query("SELECT * FROM channels WHERE link = :link LIMIT 1")
    Channel getChannelById(String link);

    @Query("SELECT * FROM channels")
    ArrayList<Channel> getAllChannels();
}

有了这些实体、DAO 和 POJO，我可以获得 Channel 对象，其中包含具有相应链接 (id) 的 Items 列表。是吗？

我的另一个问题是关于其余的 CRUD。例如。如果我想保存一个新 channel ，我可以将此语句添加到我的 ChannelDao 中吗？

@Insert(onConflict = OnConflictStrategy.REPLACE)
void createChannels(Channel... channels);

删除

@Delete
void deleteChannels(Channel... channels);

等等。那么它会从传递的 Channel 对象中创建和删除 ChannelEntities 和 ItemEntities 吗？

Answer 1

我按照@CommonsWare 的建议更新了我的类(class)。现在我有了带有@Embedded 对象的实体类:

    @Entity (tableName = "channels")
public class ChannelEntity {
    // Required channel elements
    // The name of the channel. It's how people refer to your service.
    private String title;
    // The URL of the HTML website corresponding to the channel
    @PrimaryKey
    private String link;
    //other fileds
    @Embedded
    private TextInputEntity textInputEntity;
    @Embedded
    private ImageEntity imageEntity;
    //getters and setters
}

嵌入类之一:

// Specifies a text input box displayed with the channel.
// Embedded in ChannelEntity
public class TextInputEntity {
    // Required elements
    // The label of the Submit button in the text input area.
    private String title;
    // Explains the text input aera.
    private String description;
    // The name of the text object int hte text input area.
    private String name;
    // The URL of the CGI script that processes the text input request
    private String link;
    @ColumnInfo (name = "channel_link")
    private String channelLink;
}

我为所有带有@Entity 注释的类编写了 daos(我将嵌入式类称为实体，即使它们不是实体，但我有 View 的模型类，我不想以后混淆它们)。

我这样映射关系:

// All items are optional, but at least one of title or description must be presented.
@Entity (tableName = "items"
        , foreignKeys = @ForeignKey (entity = Channel.class
        , parentColumns = "link"
        , childColumns = "channel_link"))
public class ItemEntity {
    @PrimaryKey (autoGenerate = true)
    private int id;
    // Title of the item
    private String title;
    // Other fileds
    @ColumnInfo (name = "channel_link")
    private String channelLink;
    // Getters and setters

当我想获得一个包含项目列表的 channel 时，我是这样得到的:

public class Channel {
    @Embedded
    private ChannelEntity channel;
    @Relation (parentColumn = "link", entityColumn = "channel_link")
    private ArrayList<ItemEntity> items;
    @Relation (parentColumn = "link", entityColumn = "channel_link")
    private ArrayList<SkipDayEntity> skipDays;
    @Relation (parentColumn = "link", entityColumn = "channel_link")
    private ArrayList<SkipHourEntity> skipHours;
//Setters and getters
}

这是 Channel 的 dao:

@Dao
public interface ChannelDao {
    @Insert (onConflict = OnConflictStrategy.REPLACE)
    void insertChannel(ChannelEntity channel);

    @Update
    void updateChannel(ChannelEntity channel);

    @Delete
    void deleteChannel(ChannelEntity channel);

    @Query ("SELECT * FROM channles WHERE link = :link LIMIT 1")
    Channel getChannelByLink(String link);

    @Query ("SELECT * FROM channels")
    LiveData<ArrayList<Channel>> getAllChannels();
}