Support List arguments in QuerydslDataFetcher

Closes gh-281

spring-graphql/src/main/java/org/springframework/graphql/data/query/QuerydslDataFetcher.java

@@ -130,7 +130,9 @@ public abstract class QuerydslDataFetcher<T> {
 		this.customizer.customize(bindings, path);
 
 		for (Map.Entry<String, Object> entry : environment.getArguments().entrySet()) {
-			parameters.put(entry.getKey(), Collections.singletonList(entry.getValue()));
+			Object value = entry.getValue();
+			List<Object> values = (value instanceof List ? (List<Object>) value : Collections.singletonList(value));
+			parameters.put(entry.getKey(), values);
 		}
 
 		return BUILDER.getPredicate(this.domainType, (MultiValueMap) parameters, bindings);

spring-graphql/src/test/java/org/springframework/graphql/data/query/QuerydslDataFetcherTests.java

@@ -111,6 +111,21 @@ class QuerydslDataFetcherTests {
 		tester.accept(graphQlSetup(mockRepository));
 	}
 
+	@Test
+	void shouldFetchMultipleItemsWithListInput() {
+		Book book1 = new Book(42L, "Hitchhiker's Guide to the Galaxy", new Author(0L, "Douglas", "Adams"));
+		Book book2 = new Book(53L, "Breaking Bad", new Author(0L, "", "Heisenberg"));
+		mockRepository.saveAll(Arrays.asList(book1, book2));
+
+		Mono<WebOutput> output = graphQlSetup(mockRepository).toWebGraphQlHandler()
+				.handleRequest(input("{ booksById(id: [42,53]) {name}}"));
+
+		List<String> names = GraphQlResponse.from(output).toList("booksById", Book.class)
+				.stream().map(Book::getName).collect(Collectors.toList());
+
+		assertThat(names).containsExactlyInAnyOrder(book1.getName(), book2.getName());
+	}
+
 	@Test
 	void shouldApplyCustomizerInRepository() {
 		MockWithCustomizerRepository repository = repositoryFactory.getRepository(MockWithCustomizerRepository.class);

spring-graphql/src/test/resources/books/schema.graphqls

@@ -1,5 +1,6 @@
 type Query {
     bookById(id: ID): Book
+    booksById(id: [ID]): [Book]
     books(id: ID, name: String, author: String): [Book]
     booksByCriteria(criteria:BookCriteria): [Book]
     booksByProjectedArguments(name: String, author: String): [Book]