Predicate on a field of an entity class which is type of a parent class and filter on fields of its children

Question

In the entity class A there is a field of type P1 named as p. Class P2 and P3 is extended from P1. In the predicate I want to filter the result based on a field of class P2. How can I do this?

public class A {
    private P1 p;
}

public class P1 {
}

public class P2 extends P1 {
    private String s;
}

public class P3 extends P1 {
    private String u;
}

public interface ARepository extends CrudRepository<A, Long>, QuerydslPredicateExecutor<A>, QuerydslBinderCustomizer<QA> {
    @Override
    default void customize(QuerydslBindings bindings, QA a) {
        bindings.bind(String.class).first((SingleValueBinding<StringPath, String>) StringExpression::containsIgnoreCase);
    }
    default Page<A> findByFilter(AFilter filter, Pageable pageable) {
        Predicate predicate = new APredicatesBuilder().build(filter);
        return findAll(predicate, pageable);
    }
}

class APredicatesBuilder {
    private List<BooleanExpression> predicates = new ArrayList<>();
    private addPredicate(BooleanExpression predicate) {
        if (null != predicate) {
            predicates.add(predicate);
        }
    }
    BooleanExpression build(AFilter filter) {
        QA a = QA.a;
        if (filter.getS() != null) {
            addPredicate(a.p.s.eq(filter.getS()); // This is not possible. How to do it?
        }
        BooleanExpression result = null;
        if (!predicates.isEmpty()) {
            result = predicates.get(0);
            for (int i = 1; i < predicates.size(); i++) {
                result = result.and(predicates.get(i));
            }
        }
        return result;
    }
}

class AFilter {
    private String s;
}

As I commented in the code, this is not possible to add predicate on A and filter the results based on field s in class P2.

Answer 1

This is an issue with your class hierarchy: A has a field p of type P1 but the field s does only exist in class P2. I would recomment to migrate p to type P2 or to move s to parent class P1.

At the end, this has nothing to do with querydsl and happen in the same way with pure java.