Saturday, March 2, 2024

Methods to Typesafely Map a Nested SQL Assortment right into a Nested Java Map with jOOQ – Java, SQL and jOOQ.


A extremely cool, current query on Stack Overflow was about tips on how to map a nested assortment right into a Java Map with jOOQ. Prior to now, I’ve blogged concerning the highly effective MULTISET operator many instances, which permits for nesting collections in jOOQ. This time, as a substitute of nesting information right into a Record<UserType>, why not nest it in a Map<UserType1, UserType2> as a substitute?

Wanting on the Sakila database, let’s take a look at how we will fetch information into this Java document kind:

document Movie(
    String title,
    Map<LocalDate, BigDecimal> income
) {}

The end result kind ought to wrap up the FILM.TITLE together with the amount of cash or DVD rental retailer made on every day, per movie. We may use different information constructions, however let’s assume that is what we need to eat within the UI or service or no matter.

In jOOQ, as ever so typically, due to MULTISET, we will do it in a single question that appears like this:

Record<Movie> end result =
ctx.choose(
        FILM.TITLE,
        multiset(
            choose(
                PAYMENT.PAYMENT_DATE.forged(LOCALDATE), 
                sum(PAYMENT.AMOUNT))
            .from(PAYMENT)
            .the place(PAYMENT.rental().stock().FILM_ID
                .eq(FILM.FILM_ID))
            .groupBy(PAYMENT.PAYMENT_DATE.forged(LOCALDATE))
            .orderBy(PAYMENT.PAYMENT_DATE.forged(LOCALDATE))
        )
        // Convert Subject<Outcome<Record2<LocalDate, BigDecimal>>>
        // to Subject<Map<LocalDate, BigDecimal>>
        .convertFrom(r -> r.gather(Information.intoMap())
   )
   .from(FILM)
   .orderBy(FILM.TITLE)

   // Convert Record2<String, Map<LocalDate, BigDecimal>>
   // to Record<Movie>
   .fetch(Information.mapping(Movie::new))

We will then eat the end result, e.g. like this:

for (Movie movie : end result) {
    System.out.println();
    System.out.println("Movie %s with income: "
        .formatted(movie.title()));

    // Inferred varieties are LocalDate d and BigDecimal r
    movie.income().forEach((d, r) -> 
        System.out.println("  %s: %s".formatted(d, r))
    );
}

To supply:

Movie ACADEMY DINOSAUR with income: 
  2005-05-27: 0.99
  2005-05-30: 1.99
  2005-06-15: 0.99
  [...]

Movie ACE GOLDFINGER with income: 
  2005-07-07: 4.99
  2005-07-28: 9.99
  2005-08-01: 4.99
  [...]

Every thing is, as all the time with jOOQ, fully kind protected! Attempt it your self, change a number of the column expressions within the question, or the ensuing document or Map kind to see that the question will cease compiling!

The fascinating bit right here is:

.convertFrom(r -> r.gather(Information.intoMap())

The Subject.convertFrom() methodology is from jOOQ 3.15’s new ad-hoc conversion API, which permits for ad-hoc changing a Subject<T> column expression to a Subject<U> column expression. On this case, the conversion goes:

  • From Subject<Outcome<Record2<LocalDate, BigDecimal>>> (the multiset subject kind)
  • To Subject<Map<LocalDate, BigDecimal>> (the mapped kind)

It does so by amassing all of the Record2<LocalDate, BigDecimal> data of the nested assortment right into a Map<LocalDate, BigDecimal> utilizing the Information.intoMap() collector. The signature of that methodology is:

public static remaining <Ok, V, R entends Record2<Ok, V>> 
Collector<R, ?, Map<Ok, V>> intoMap() { ... }

That particular utilization of generics permits for avoiding the repetition of the important thing and worth expressions of the sphere, understanding {that a} assortment of Record2<Ok, V> has an apparent strategy to gather right into a Map<Ok, V> (or Map<Ok, Record<V>> in the event you’re utilizing Information.intoGroups(), if keys will be duplicate).

Notice that each of those collectors will produce an insertion order preserving Map (e.g. LinkedHashMap), such that any MULTISET ordering can be preserved.

Conclusion

The sky is the restrict, once you’re utilizing jOOQ 3.15’s new nesting capabilities for nested collections (MULTISET or MULTISET_AGG) or nested data (ROW). Along with ad-hoc converters, you may map the jOOQ illustration into any Java illustration in the midst of your question, to stick to any goal kind of your selecting, together with nested Map<Ok, V>, with arbitrary forms of Ok and V

Related Articles

LEAVE A REPLY

Please enter your comment!
Please enter your name here

Latest Articles