How to work with nested relationships in Room

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I have entities:

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

@Entity()
public class B {
    @PrimaryKey @NonNull
    public String id;
    public String oneCId;
    public String anotherCId;
    public long aId;
    public B() {}
}

@Entity
public class C {
    @PrimaryKey @NonNull
    public String id;
    public String value;
    public C() {}
}

and some POJOs:

public class AWithB {
    @Embedded
    public A a;

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

    public AWithB() {}
}

public class BWithC {
    @Embedded
    public B b;
    public C oneC;
    public C anotherC;

    public BWithC() {}
}

with DAO:

@Query("SELECT * FROM a")
List<AWithB> getAllNow();

The problem is with the @Relation for AWithB as it cannot point to anything else than entity. But that entity cannot include other entities. How should I return the whole structure from DB?

database-relations android-architecture-components android-room sqlite android
1 Answers

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() {}
}
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