（房间数据库）查询错误未返回的字段

Question

这是我的代码：

ClassEntity.java

@Entity

public class ClassEntity {

    @NonNull
    @PrimaryKey
    public String id_of_a_group;
    public String monday;
    public String tuesday;
    public String wednesday;
    public String thursday;    
    public String friday;
}

Dao.java

@androidx.room.Dao    
public interface Dao {

    @Query("SELECT monday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<ClassEntity> findFromMonday(String id);

    @Query("SELECT tuesday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<ClassEntity> findFromTuesday(String id);

    @Query("SELECT wednesday FROM  ClassEntity WHERE id_of_a_group = :id")   
    List<ClassEntity> findFromWednesday(String id);

    @Query("SELECT thursday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<ClassEntity> findFromThursday(String id);

    @Query("SELECT friday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<ClassEntity> findFromFriday(String id);

    @Query("SELECT id_of_a_group FROM  ClassEntity")
    List<ClassEntity> getIdOfAllGroups();

    @Insert
    void insert(Class group);

    @Update
    void update(Class group);

    @Delete
    void delete(Class group);
}

Database.java

@androidx.room.Database(entities = {ClassEntity.class},version=1)

public abstract class Database extends RoomDatabase {

    public abstract Dao classDao();   
    public abstract Array[] getIdOfAllGroups();
}

App.java

public class App extends Application {

    public static App instance;    
    private Database database;

    @Override
    public void onCreate() {

        super.onCreate();
        instance = this;
        database = Room.databaseBuilder(this, Database.class, "database")
                .build();
    }

    public static App getInstance() {
        return instance;
    }

    public Database getDatabase() {
        return database;
    }
}

编译器给了我这个错误：

ClassEntity has some fields [id_of_a_group, tuesday, wednesday, thursday, friday] which are not returned by the query. If they are not supposed to be read from the result, you can mark them with @Ignore annotation. You can suppress this warning by annotating the method with @SuppressWarnings(RoomWarnings.CURSOR_MISMATCH). Columns returned by the query: monday. Fields in com.example.schedule.ClassEntity: id_of_a_group, monday, tuesday, wednesday, thursday, friday.

我不知道解决方案，因为我是新手。 顺便说一句，我需要检查数据库中是否有该项目。

这是我的猜测，如果我错了，请告诉我

    Others2 activity=(Others2)getActivity();
    getData=activity.sendData();
    Database db = App.getInstance().getDatabase();
    String[] ids_of_all_groups=db.getIdOfAllGroups();
    boolean test=false;

    for( String elem: ids_of_all_groups){
        if(getData==elem){
            test=true;
            break;
        }
    }

Answer 1

您正试图告诉 Room 从单个字符串（对于每个查询）构建一个 ClassEntity 对象。它是说它不能安全地做到这一点。

解决方法是将List<String> 与List<calssEntity> 相对应。

当您想要基于 ClassEntity 对象插入/更新/删除时，您还尝试使用 Class 对象插入/删除/更新。所以你的道应该/可以是：-

@androidx.room.Dao
public interface Dao {

    @Query("SELECT monday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<String> findFromMonday(String id);

    @Query("SELECT tuesday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<String> findFromTuesday(String id);

    @Query("SELECT wednesday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<String> findFromWednesday(String id);

    @Query("SELECT thursday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<String> findFromThursday(String id);

    @Query("SELECT friday FROM  ClassEntity WHERE id_of_a_group = :id")
    List<String> findFromFriday(String id);

    @Query("SELECT id_of_a_group FROM  ClassEntity")
    List<String> getIdOfAllGroups();

    @Insert
    void insert(ClassEntity group);

    @Update
    void update(ClassEntity group);

    @Delete
    void delete(ClassEntity group);
}

您还应该注释掉或删除该行（在 Database.java 中）

public abstract Array[] getIdOfAllGroups();

我相信以上的改动将允许编译。

至于检查数据库中的项目，您可以通过将以下内容添加到 Dao.java 来添加查询以检查 ClassEntity 表中是否有任何行

@Query("SELECT count() FROM ClassEntity")
int getItemsInDatabase();

使用上述更改，然后在活动中使用以下更改：-

    database = Room.databaseBuilder(this, Database.class, "database")
            .allowMainThreadQueries()
            .build();
    Log.d("ITEMSINDB",String.valueOf(database.classDao().getItemsInDatabase()));
    if (database.classDao().getItemsInDatabase() < 1) {
        ClassEntity myClassEntity = new ClassEntity();
        myClassEntity.id_of_a_group = "myfirstid";
        myClassEntity.monday = "something on Monday";
        myClassEntity.tuesday = "on Tuesday";
        myClassEntity.wednesday = "another on Wed";
        myClassEntity.thursday = "this on Thurs";
        myClassEntity.friday = "TGIF";
        database.classDao().insert(myClassEntity);
    }
    Log.d("ITEMSINDB",String.valueOf(database.classDao().getItemsInDatabase()));

结果：-

2020-01-07 09:48:30.251 D/ITEMSINDB: 0
2020-01-07 09:48:30.254 D/ITEMSINDB: 1

如果重新运行，则由于数据存在，结果是（行已经存在，因此没有添加新行）

2020-01-07 09:57:03.217 D/ITEMSINDB: 1
2020-01-07 09:57:03.218 D/ITEMSINDB: 1

关于public abstract Array[] getIdOfAllGroups();，您将使用（在应用活动中）：-

List<String> mygroups = database.classDao().getIdOfAllGroups();

---

如果你想根据它的 id_of_a_group 检查一个组是否存在，那么你可以有

@Query("SELECT count() > 0  FROM ClassEntity WHERE id_of_a_group = :id")
boolean doesGroupExist(String id);

你可以使用类似的东西：-

    String test1 = "this group doesn not exists";
    if (!database.classDao().doesGroupExist(test1)) {
        Log.d("GROUPLOOKUPRESULT","Group " + test1 + " Not located.");
    } else {
        Log.d("GROUPLOOKUPRESULT","Group " + test1 + " Located.");
    }
    String test2 = "myfirstid";
    if (database.classDao().doesGroupExist(test2)) {
        Log.d("GROUPLOOKUPRESULT","Group " + test2 + " Located.");
    } else {
        Log.d("GROUPLOOKUPRESULT","Group " + test2 + " Not Located.");
    }

这会导致：-

2020-01-07 10:17:49.201 D/GROUPLOOKUPRESULT: Group this group doesn not exists Not located.
2020-01-07 10:17:49.201 D/GROUPLOOKUPRESULT: Group myfirstid Located.