select p.id
from post p
  inner join jhi_user contact ON p.created_by = contact.login;

private Specification<Post> createSpecification(PostCriteria criteria) {
        return (root, query, cb) -> {
            Specification<Post> s = Specification.where(null);

            Join<Post, User> contact = root.join("jhi_user", JoinType.INNER);
            contact.alias("contact");
            contact.on(cb.equal(contact.get(User_.login), root.get(Post_.createdBy)));

            root.joinSet(, JoinType.INNER);

            return s.toPredicate(root, query, cb);
        };
    }

org.springframework.dao.InvalidDataAccessApiUsageException: Unable to locate Attribute  with the the given name [jhi_user] on this ManagedType [*.domain.AbstractAuditingEntity]; nested exception is java.lang.IllegalArgumentException: Unable to locate Attribute  with the the given name [jhi_user] on this ManagedType [*.domain.AbstractAuditingEntity]

root.joinSet(, JoinType.INNER);