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 {
    @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?

In case anyone is interested in the answer, check out this [Answer](https://stackoverflow.com/a/52554115/4980713) — Edy Daoud, Dec 12 '18 at 12:17

score 31 · Accepted Answer · answered Sep 26 '17 at 09:50

31

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() {}
}

answered Sep 26 '17 at 09:50

Andrzej Sawoniewicz

1,541
2
16
18

Hello Andrej, Do you know how to order by the AwithB for example? – Alfonso Fernandez-Ocampo Oct 13 '17 at 10:14
I am not sure what do you mean. For the A table you can use normal query with `Order By` statement as well. For ordering the list of entities you got from @Relation you have to do it by yourself in your getter in your Java Bean (AWithB::getBWithCOrdered) for now at least. – Andrzej Sawoniewicz Oct 17 '17 at 08:19
1

The key here is to use the POJO as the `set` type and NOT in `@Relation(entity =..`, which seems obvious but for some reason, I though I had to. If you're going to specify the entity just like in the first example: **use the entity in the annotation and the POJO as the set / list type.** – FirstOne Sep 16 '18 at 01:44

How to work with nested relationships in Room

1 Answers1

Linked