Android room DAO interface does not work with inheritance

Question

interface Marker<T : BaseFoo> {
    fun getSpecialFoo(): List<T>
}

@Dao
interface FooDao: Marker<Foo> {
    @Query("SELECT * FROM foo WHERE bar = :bar")
    fun get(bar: Int): List<Foo>

    @Transaction
    override fun getSpecialFoo(): List<Foo> {
        return get(1)
    }
}

This results in

An abstract DAO method must be annotated with one and only one of the following annotations: Insert,Delete,Query,Update,RawQuery

However, Marker is not marked with @Dao and FooDao override getSpecialFoo already. Why does this error still show?

I need Marker because I need a generic DAO that has some methods. Is there a way to work around this?

The only way I can think of is marking the dao to Any and cast the type runtime or build a wrapper for the DAO.

Answer 1

This seems like it is limitation of the Room library. You can work around it like the following.

@Dao
interface FooDao {
    @Query("SELECT * FROM foo WHERE bar = :bar")
    fun get(bar: Int): List<Foo>

    @Transaction
    fun getSpecialFoo(): List<Foo> {
        return get(1)
    }
}

fun FooDao.wrapper(): Marker<Foo> {
    return Wrapper(this)
}


private class Wrapper(private val dao: FooDao): Marker<Foo> {
    override fun getSpecialFoo() = dao.getSpecialFoo()
}

When you need it to be Marker<Foo>, you can wrapper() to create a wrapper which implement Marker<Foo> by the dao.