如果项目存在于数据库中，为什么 Room 查询会返回 null？

Question

I have inserted a Movie into my database and want to query the database based on the movieId. I can confirm that the movieId is in my database using the Stetho inspector. Why am I getting null?

DAO

@Dao
public interface MoviesDao {

    @Insert(onConflict = OnConflictStrategy.IGNORE)
    void insertMovie(Movie movie);

    @Transaction
    @Query("SELECT * FROM movie WHERE id = :movieId ")
    LiveData<Movie> getMovie(long movieId);

ViewModel

    public void getMovieFromDatabase(Movie movie) {
        LiveData<Movie> result = movieRepository.getMovieFromDatabase(movie);
        Log.d(TAG, "result is: " + result.getValue());
    }

Repository

    public LiveData<Movie> getMovieFromDatabase(Movie movie) {
        Log.d(TAG, "Movie id: " + movie.getId());
        return dbInstance.moviesDao().getMovie(movie.getId());
    }

Result from Logcat

Movie id: 419704
result is: null

Image of database. Clearly movie id 419704 exists (second item):

Answer 1

getMovie() is your DAO method. It returns a LiveData . That will cause Room to do the database I/O on a background thread.

However, you immediately attempt to get the value out of the LiveData . That will not work most of the time, as the background thread will not have completed its work yet.

Either:

• Get rid of the LiveData and have getMovie() return the results directly, and call getMovie() on a background thread, or

• observe() the LiveData , rather than attempt to use its value immediately