Room DAO 无法从数据库检索数据

Question

I have a "Contacts" table and a "Phone Numbers" table. The table with the phone numbers includes a row which points to the autogenerated ID of the contact. I try to retrieve all phone numbers linked to that contact in my PhoneNumbersDao, like below:

@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);
}

My PhoneNumber class:

@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;
    }
}

And lastly, I try to access the database using this method in PhoneNumberRepository:

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

But the method still returns null. Why could that be so?

Answer 1

Do add this observer in your Fragment or Activity via this observer you will be able to get data from the database if you have data in the database otherwise you will get null

 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
      }
  });

Answer 2

You should have two constructors in your PhoneNumber class, one that takes the id, and another without it with the ignore annotation.Maybe that is the reason the data is not being saved in your databse. So your code should be like

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

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

Answer 3

Room required default constructor. So create default constructor in your entity.

 public PhoneNumber() {
 }