Android Room one-to-one relation (getting foreign fields) [duplicate]

Question

I want to implement android ROOM to my project. But I can not solve one-to-one relation between Order(customer_id) and Customer(id).

Is it possible to fill customer (@Ignore protected Customer customer) field in OrderDao interface?

I can solve only java codes. Is there any other methods?

//    Customer.java 
@Entity (tableName = "t_customer")
public class Customer {
  @PrimaryKey(autoGenerate = true)
  @ColumnInfo(name = "id")
  protected String mId;

  @ColumnInfo(name = "name")
  protected String mName;

}

// Order.java
@Entity(tableName = "t_order",
    foreignKeys = {
    @ForeignKey(entity = Customer.class,
        parentColumns = "id",
        childColumns = "customer_id",
        onDelete = SET_NULL) }
public class Order {
  @PrimaryKey(autoGenerate = true)
  @ColumnInfo(name = "id")
  protected String mId;

  @ColumnInfo(name = "description")
  protected String mDescription;

  @ColumnInfo(name = "customer_id")
  protected String mCustomerId;

  @Ignore
  **protected Customer customer;**

//....  
}

//OrderDao.java

@Dao
public interface OrderDao {

  @Query("SELECT * from t_order ORDER BY id DESC")
  LiveData<List<Order>> listByIds();

  @Query("SELECT * from t_order WHERE id = :id LIMIT 1")
  LiveData<Order> readById(long id);

//....
}

Answer 1

As of right now there is no way to fill Customer automatically. You'll need to create another POJO named OrderWithCustomer and it will probably looks as shown below:

public class OrderWithCustomer {

    @Embedded
    private Order order;

    @ColumnInfo(name = "name")
    private String customerName;
}

And in your DAO:

@Dao
interface OrderDao {

    @Query("SELECT t_order.* name FROM t_order LEFT JOIN t_customer ON t_order.customer_id = t_customer.id")
    public List<OrderWithCustomer> getOrderWithCustomer();

}