java 43 lines · 8 steps

Solving GraphQL N+1 with batch mapping in Spring

A Spring GraphQL controller resolves queries, batches author lookups to avoid N+1, and computes a derived field.

Explained by highlit

Spring Intermediate graphql n+1-problem batching dependency-injection streams

1@Controller
2public class BookController {
3 
  private final BookService bookService;
  private final AuthorService authorService;
6 
  public BookController(BookService bookService, AuthorService authorService) {
      this.bookService = bookService;
      this.authorService = authorService;
  }
11 
  @QueryMapping
  public Book bookById(@Argument String id) {
      return bookService.findById(id)
              .orElseThrow(() -> new BookNotFoundException(id));
  }
17 
  @QueryMapping
  public Page<Book> books(@Argument int page, @Argument int size, @Argument String genre) {
      return bookService.search(genre, PageRequest.of(page, size));
  }
22 
  @BatchMapping(typeName = "Book", field = "author")
  public Map<Book, Author> author(List<Book> books) {
      Set<String> authorIds = books.stream()
              .map(Book::getAuthorId)
              .collect(Collectors.toSet());
28 
      Map<String, Author> authorsById = authorService.findAllById(authorIds).stream()
              .collect(Collectors.toMap(Author::getId, Function.identity()));
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      return books.stream()
              .filter(book -> authorsById.containsKey(book.getAuthorId()))
              .collect(Collectors.toMap(
                      Function.identity(),
                      book -> authorsById.get(book.getAuthorId())));
  }
38 
  @SchemaMapping(typeName = "Book")
  public String displayTitle(Book book) {
      return book.getTitle() + " (" + book.getPublicationYear() + ")";
  }
43}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Batch mapping resolves a nested field for many parents in one call, eliminating per-record N+1 queries.
2Building a lookup map keyed by id turns repeated service calls into a single bulk fetch plus in-memory joins.
3Schema mappings let you expose computed fields that don't exist on the underlying entity.

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