@SessionFactory(exportSchema = false)
@DomainModel(annotatedClasses = {
    TestStructNestedFieldQuery.StructHolder.class,
    TestStructNestedFieldQuery.SomeStruct.class
})
public class TestStructNestedFieldQuery {

    @Test
    void test(SessionFactoryScope scope) {
        scope.inTransaction(session -> {
            var holder = new StructHolder();
            holder.id = 1;
            holder.struct = new SomeStruct();
            holder.struct.firstName = "John";
            holder.struct.lastName = "Doe";
            session.persist(holder);
        });

        scope.inTransaction(session -> {
            var query = session.createSelectionQuery("SELECT s FROM StructHolder s WHERE s.struct.firstName = :firstName", StructHolder.class);
            query.setParameter("firstName", "John");
            query.getResultList();
        });
    }
    @Entity(name = "StructHolder")
    @Table(name = "collection")
    static class StructHolder {
        @Id
        int id;

        SomeStruct struct;
    }

    @Embeddable
    @Struct(name = "someStruct")
    static class SomeStruct {
        String firstName;
        String lastName;
    }
} 

{"aggregate": "collection", "pipeline": [{"$match": {"unused from MongoAggregateSupport.aggregateComponentAssignmentExpression": {"$eq": {"$undefined": true}}}}, {"$project": {"_id": true, "struct": true}}]} 

{"aggregate": "collection", "pipeline": [{"$match": {"struct.firstname": {"$eq": {"$undefined": true}}}}, {"$project": {"_id": true, "struct": true}}]}