How to use joins in JPQL constructor expressions?

I have a data model with Customers, which each have n Portfolios, which in turn have n Investments, both mapped as @ManyToMany and lazily fetched.

I also have a DTO with just the following fields:

• customer name
• list of portfolio names

Is it possible to write a single JPQL query that creates the DTOs using a constructor expression? Especially, how would you get a list of portfolio names in the DTO constructor?

Is it inefficient if I instead query for Customer models with portfolios and construct the DTOs myself?

You'll have to do it by yourself, and it will probably even be slightly more efficient than using a constructor in JPQL, since reflection won't be necessary:

select c.name, p.name from Customer c left join c.portfolios

Then loop over the results, and construct your results. For example:

Map<String, Result> results = new HashMap<String, Result>();
for (Object[] row : rows) {
    String customerName = (String) row[0];
    Result r = results.get(customerName);
    if (r == null) {
        result.put(customerName, new Result(customerName));
    }
    r.addPortfolio((String) row[1]);
}
Collection<Result> namesAndPortfolios = results.values();