How to work with nested relationships in Room

Question

I have entities:

@Entity
public class A {
    @PrimaryKey(autoGenerate = true)
    public long id;
    public A() {}
}

@Entity()
public class B {
    @PrimaryKe         


        

Answer 1

It seems that you can have nested relations (the Javadoc on documentation page is for some reason not showing the whole code and is misleading for that reason).

It is working:

public class AWithB {
    @Embedded
    public A a;

    @Relation(parentColumn = "id", entityColumn = "aId", entity = B.class)
    public List<BWithC> bWithC;

    public AWithB() {}
}

For relations Many To One you can still use @Relation annotation. For some reason you cannot have simple instance - you need a collection here. But it is working:

public class BWithC {
    @Embedded
    public B b;
    @Relation(parentColumn = "oneCId", entityColumn = "id")
    public Set<C> oneC;
    @Relation(parentColumn = "anotherCId", entityColumn = "id")
    public Set<C> anotherC;

    public BWithC() {}
}