Android 房间 onDelete 级联无法正常工作

Question

所以我的这个数据库有一个 Deck table、一个 Card table 和一个 DeckCard table 因为这两者之间存在多对多的关系.. .当我尝试删除牌组时，我删除了 ID 匹配的 DeckCard 行，并且我设置了外键 onDelete = CASCADE 但它没有删除牌组的任何建议？这是我 n 到 n 区别的存储库

public class DeckCardRepository {
    private DeckWithCardsDao mDeckWithCardsDao;
     public void delete(Deck deck){
        DecksDatabase.databaseWriteExecutor.execute(() -> 
         mDeckWithCardsDao.delete(deck.getId()));
    }

}

甲板class

@Entity(tableName = "decks")
public class Deck implements Serializable {
    @PrimaryKey
    private long id;
    private String keyforgeId;
    private String name;
    @TypeConverters({ExpansionTypeConverter.class})
    private Expansion expansion;
    private int creatureCount;
    private int actionCount;
    private int artifactCount;
    private int upgradeCount;
    private int sasRating;
    private int powerLevel;
    private int chains;
    private int wins;
    private int losses;
    private int totalPower;
    private int totalArmor;
    private int localWins;
    private int localLosses;
}

卡片class

Entity(tableName = "cards")
public class Card implements Serializable {
    @PrimaryKey
    @NonNull
    private String id;
    private String card_title;
    private String card_type;
    @TypeConverters({HouseArrayTypeConverter.class})
    private House house;
    private String card_text;
    private int amber;
    private String front_image;
}

卡片组class

Entity(tableName = "cards_deck_join",
        primaryKeys = {"cardId", "deckId"},
        foreignKeys = {
                @ForeignKey(
                        entity = Card.class,
                        parentColumns = "id",
                        childColumns = "cardId"),
                @ForeignKey(onDelete = CASCADE,
                        entity = Deck.class,
                        parentColumns = "id",
                        childColumns = "deckId"),
        })
public class CardsDeckRef {
    @NonNull
    private String cardId;
    private long deckId;
    private int count;

    public CardsDeckRef(String cardId, long deckId, int count) {
        this.cardId = cardId;
        this.deckId = deckId;
        this.count = count;
    }
}

MyDao

@Dao
public interface DeckWithCardsDao {
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    void add(CardsDeckRef cardsDeckRef);

    @Transaction
    @Query("SELECT * FROM cards INNER JOIN cards_deck_join" +
            " ON cards.id=cards_deck_join.cardId WHERE cards_deck_join.deckId =:deckId")
    LiveData<List<Card>> getCardsForDeck(final long deckId);

    @Query("DELETE FROM cards_deck_join WHERE cards_deck_join.deckId=:deckId ")
    void delete(final long deckId);

}

Answer 1

如果 parent 被删除，

ON CASCADE DELETE（即 onDelete = CASCADE 添加到 table）将删除 children。如果 child 甚至所有 children 都被删除，它不会删除 parent。

而不是删除牌组和所有与牌组关联的 cards_deck_join 行，您应该使用

@Query("DELETE FROM decks WHERE id=:deckId ")
void delete(final long deckId);

牌组的删除将向下级联到 cards_deck_join table。

如果你想在从 cards_deck_join 删除后没有 children 的情况下自动删除牌组，那么你可以使用 TRIGGER 例如

CREATE TRIGGER IF NOT EXISTS delete_empty_deck 
    AFTER DELETE ON card_deck_join
    BEGIN
        DELETE FROM decks 
            WHERE (SELECT count(*) FROM card_deck_join WHERE deckid = old.deckid) = 0 AND decks.id = old.deckid;
    END;

由于 room 不支持触发器的生成（据我所知，FTS 除外，它确实会生成一些触发器），您必须在 room 之外添加 TRIGGER（例如通过覆盖 onCreate 或 OnOpen 的回调）。

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