Room query return LiveData of parent class type

Question

I'm having three clases:

• Task (parent class)
• ProgressTask (subclass of Task)
• TodolistTask (subclass of Task)

ProgressTask and TodolistTask differ by few parameters.

Both ProgressTask and TodolistTask are entities in my Room db. I want to have TaskDao in which I would like to call this query:

@Query("SELECT * FROM  progressTask_table UNION SELECT * FROM todolistTask_table")
LiveData<List<Task>> getAllTasks();

is it possible? Now I'm getting a reasonable error saying that SELECTs to the left and right have different number of columns.

Answer 1

You have to call out the individual columns in you query when using a union.

You objects most likely have different columns. Example ProgressTask has a date that TodolistTask doesn't.

If you call out each column in the base class in both select parts of your query it will work.