jpa 2.0 - Spring JPA Specification Join Two Tables Only to filter the Query

Question

I have been Trying to generate JPA2 Specification that can filter data by joining another table. The tables are

ApplicationUser{
  String userId;
}

ExternalUser{
  String userId;
}

Comment{
  String message;
  String userType; //appuser or externaluser
  String userId; // Either the user id in ApplicationUser table or ExternalUser table
  
}

Now I would like to run a query to find all Comments posted by ExternalUser with this query

select comment.* from Comment inner outer join ExternalUser on (ExternalUser.userId = Comment.userId and Comment.userType = 'externaluser')

How to query this using Jpa2 Specification wihout the User table actually having a OneToMany Mapping. The user class is already fixed so cannot change it to add the relationship.

public default Specification<Comment> search(String userType,String userId) {
  Specification<Comment> spec = (root, cq, cb) -> {
    return predicate???
  }
}

Answer 1

等待大神答复