java - Room DAO 无法从数据库检索数据

Question

我有一个“联系人”表和一个“电话号码”表。包含电话号码的表包含一行，该行指向联系人自动生成的 ID。我尝试在 PhoneNumbersDao 中检索链接到该联系人的所有电话号码，如下所示:

@Dao
public interface PhoneNumberDao {

    @Insert
    void insert(PhoneNumber phoneNumber);

    @Update
    void update(PhoneNumber phoneNumber);

    @Delete
    void delete(PhoneNumber phoneNumber);


    // Retrieve entry/entries by contact ID
    @Query("SELECT * FROM phone_numbers_table WHERE contact_id =:contactId")
    LiveData<List<PhoneNumber>> getPhoneNumbersById(long contactId);
}

我的电话号码类:

@Entity(tableName = "phone_numbers_table")
public class PhoneNumber implements Serializable {

    @PrimaryKey(autoGenerate = true)
    public int id;
    /** String resource ID for the phone number */
    @SerializedName("phone_number")
    public String mPhoneNumber;
    /** String resource ID for the phone number type */
    @SerializedName("phone_number_type")
    public String mPhoneNumberType;
    /** String resource ID for the respective contact ID */
    @SerializedName("contact_id")
    @ColumnInfo(name = "contact_id")
    public int contactId;

    public PhoneNumber(String phoneNumber, String phoneNumberType, int contactId) {
        this.mPhoneNumber = phoneNumber;
        this.mPhoneNumberType = phoneNumberType;
        this.contactId = contactId;
    }

    public int getId() {
        return id;
    }

    public String getPhoneNumber() {
        return mPhoneNumber;
    }

    public String getPhoneNumberType() {
        return mPhoneNumberType;
    }

    public int getContactId() {
        return contactId;
    }
}

最后，我尝试在 PhoneNumberRepository 中使用此方法访问数据库:

public LiveData<List<PhoneNumber>> getPhoneNumbersByContactId(long id) {
        return phoneNumberDao.getPhoneNumbersById(id);
    }

但是该方法仍然返回 null。为什么会这样？

Answer 1

请将此观察者添加到您的Fragment或Activity中，通过此观察者，如果数据库中有数据，您将能够从数据库获取数据，否则您将得到空

 viewModel.getPhoneNumbersByContactId(contactId).observe(this, new Observer<List<PhoneNumber>>() {
      @Override
      public void onChanged(@Nullable List<PhoneNumber> phoneList) {
//Here you will get all your phone list if you have any in database else you will get null
      }
  });