EntityGraphFactory overwrites nested subgraphs

[pstrnad]

I'm trying to eager join OneToMany mappings of three entities, e.g.:

class A {
    int id;

    @OneToMany
    Set<B> b;
}
class B {
    int id;

    @OneToMany
    Set<C> c;
}
class C {
    int id;
}

When fetching an object of class A, the EntityGraphFactory.create method receives in the "properties" parameter a set of:
"b.c.id",
"b.id",
"id"
As it processes the "b.c.id" property, it adds a subgraph for B to the entityGraph, then adds a subgraph for C to the subgraph for B.
However, when it proceeds to the "b.id" property, it adds a new subgraph for B to the entityGraph and overrides the previous one. Instead, it should check if the entityGraph already contains a subgraph and enrich it.

I ran into this when integrating Spring Data JPA with Spring GraphQL, and noticed that a GraphQL query

{
    a {
        b {
            c {
                id
            }
        }
    }
}

correctly generates a single SQL query with joins, but a query

{
    a {
        b {
            id,
            c {
                id
            }
        }
    }
}

does not, and instead triggers N+1 lazy loads

[gregturn]

Backported to 3.0.x and 2.7.x