N+1 queries when fetching N→1 relationship (Example: Address → Person) using JDO

[t-perez-r]

When querying a list of Address entities in JDO (DataNucleus), each Address references a Person (N→1 relationship).
However, querying for Person through Address causes an additional SQL query for each element in the result list, leading to an N+1 query problem.

Example models:

@PersistenceCapable
public class Address {
    @PrimaryKey Long id;
    private String street;
    private Person person; // N -> 1
}

@PersistenceCapable
public class Person {
    @PrimaryKey Long id;
    private String name;
}

Problem:

When executing a query like:

Query<?> query = pm.newQuery(Adress.class);
query.setResult("distinct person");
List<?> results = (List<?>) q.execute();
for (Object obj : results) {
    Person pers = (Person) obj;
    NucleusLogger.GENERAL.info("Person: id=" + pers.getId() + " name=" + pers.getName()); // Triggers additional query
}

It results in one query for the list, and one extra query per each element.

Notes:

• Using a fetch plan does not prevent the extra queries.
• In my application, all the necessary fields are already available, but it still issues a database query when accessing the related entity.