Use ResultQuery.collect() to Implement Powerful Mappings

In our opinion, any Iterable<T> should offer a <R> collect(Collector<T, ?, R>) method to allow for transforming the the content to something else using standard JDK collectors, jOOλ collectors from org.jooq.lambda.Agg or your own.

When using jOOQ, you don’t have to wait for the JDK to finally add these useful utilities to the Iterable API. jOOQ’s ResultQuery<R> already implements Iterable<R>, and offers additional convenience like collect() on top of it.

For example, using a Java 16 record type:

record Book (int id, String title) {}

List<Book> books =, BOOK.TITLE)
        r -> r.into(Book.class),

There are other ways to map things, but why not use a Collector. The best thing about Collector types is, they compose, type safely, and arbitrarily, almost like Stream pipelines.

I found a very interesting use-case recently on Stack Overflow. The problem there was that fetchGroups() is quite simple and not left-join aware, meaning that when an AUTHOR (parent) has no BOOK (child), instead of an empty list, there will be a list with a single NULL item:

Map<AuthorRecord, List<BookRecord>> result =
   .fetchGroups(AUTHOR, BOOK);

The above works well for inner joins, but it doesn’t really make sense for left joins. We should fix this in jOOQ, of course (, but using Collectors, you can already work around this problem today.

Simply write

Map<AuthorRecord, List<BookRecord>> result =
        r -> r.into(AUTHOR), 
            r -> r.get(BOOK.ID) != null, 
                r -> r.into(BOOK), 

// All assuming the usual static imports:
import static org.jooq.impl.DSL.*;
import static com.example.generated.Tables.*;
import static*;

Step by step:

  1. Group results by AUTHOR, mapping keys to AuthorRecord just like jOOQ’s fetchGroups()
  2. For each AUTHOR filter out those BOOK records whose BOOK.ID is null. Given that BOOK.ID is the primary key, the only reason why it could be null is because of the left join
  3. Map values to BookRecord, just like jOOQ’s fetchGroups()
  4. Collect the child records into a list.

And you’re done. Just like when you use the ResultQuery as an Iterable in a foreach loop, the collect() call executes your query automatically, managing all resources, bypassing the intermediate Result data structure, which isn’t needed here.

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