How to fill non column field of entity in room library

Question

I have a class entity as below

@Entity public class Task {         private String name;      private Integer ParentId;     private Integer userId;        @Ignore     private int noOfSubTask; } 

in DAO class there is a method getTaskList()

@Dao public interface TaskDao extends Dao<Task> {  @Query("SELECT *,(SELECT count(*) FROM Task b  WHERE a._id = b.ParentId ) AS noOfSubTask FROM Task a ") LiveData<List<Task>> getTaskList(); } 

I want to fill noOfSubTask with the number given by (SELECT count(*) FROM Task b WHERE a._id = b.ParentId ) portion of query, but problem is it is not a column so room library does not map it getTaskList method in dao implementation (auto-generated) class.

Is there any way to fill non column field of entity (like noOfSubTask in my case) using any method of dao class of room library?

Answer 1

I faced the same problem a few days ago and had a look at this thread. I am using Kotlin data classes for creating database entities. Given that Kotlin data classes do not play well with inheritance subclassing was not a viable option. My solution was to use the @Embedded keyword in a wrapping class, as follows:

data class TaskToDisplay(@Embedded                          var task: Task,                          var noOfSubTask: Int = 0) 

This solution does not create an additional column in the database and most importantly matches all Task's fields with Room's response columns.