DATAREST-1356 - Fix updating single associations.

Correct a regression regarding association links. Introduce several test cases to prove this situation is fixed regarding PUTs for association links.

Source of the bug: 554d6cb27b (diff-1d7c16fe1992fef13a47fa8ab8599718L317) flips the criteria from "not single" to "single" without checking for related impacts.

spring-data-rest-tests/spring-data-rest-tests-core/src/test/java/org/springframework/data/rest/tests/AbstractWebIntegrationTests.java

@@ -28,7 +28,6 @@ import java.util.Optional;
 
 import org.junit.Before;
 import org.junit.runner.RunWith;
-
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.data.rest.webmvc.config.RepositoryRestMvcConfiguration;
 import org.springframework.hateoas.Link;
@@ -61,6 +60,7 @@ import com.jayway.jsonpath.JsonPath;
  * @author Oliver Gierke
  * @author Greg Turnquist
  * @author Christoph Strobl
+ * @author Ľubomír Varga
  */
 @RunWith(SpringRunner.class)
 @WebAppConfiguration
@@ -115,6 +115,17 @@ public abstract class AbstractWebIntegrationTests {
 		return StringUtils.hasText(response.getContentAsString()) ? response : client.request(link);
 	}
 
+	protected MockHttpServletResponse putOnlyExpect5XXStatus(Link link, Object payload, MediaType mediaType)
+			throws Exception {
+
+		String href = link.isTemplated() ? link.expand().getHref() : link.getHref();
+
+		MockHttpServletResponse response = mvc.perform(put(href).content(payload.toString()).contentType(mediaType))//
+				.andExpect(status().is5xxServerError()).andReturn().getResponse();
+
+		return StringUtils.hasText(response.getContentAsString()) ? response : client.request(link);
+	}
+
 	protected MockHttpServletResponse patchAndGet(Link link, Object payload, MediaType mediaType) throws Exception {
 
 		String href = link.isTemplated() ? link.expand().getHref() : link.getHref();

spring-data-rest-tests/spring-data-rest-tests-jpa/src/test/java/org/springframework/data/rest/webmvc/jpa/JpaWebTests.java

@@ -61,6 +61,7 @@ import com.jayway.jsonpath.JsonPath;
  * @author Oliver Gierke
  * @author Greg Turnquist
  * @author Mark Paluch
+ * @author Ľubomír Varga
  */
 @Transactional
 @ContextConfiguration(classes = JpaRepositoryConfig.class)
@@ -321,6 +322,40 @@ public class JpaWebTests extends CommonWebTests {
 		assertSiblingNames(frodoSiblingLink, "Merry", "Pippin");
 	}
 
+	/**
+	 * Test does simulate changing creator association from Order to Person. First it sets Frodo Baggins as creator and
+	 * checks for its first name. Than it sets Pippin Baggins as creator of the Order. Check for first name is done again.
+	 */
+	@Test // DATAREST-1356
+	public void associateCreatorToOrderWithSinglePut() throws Exception {
+
+		Link firstCreatorLink = preparePersonResource(new Person("Frodo", "Baggins"));
+		Link secondCreatorLink = preparePersonResource(new Person("Pippin", "Baggins"));
+		Link orderLinkToItsCreator = prepareOrderResource(new Order());
+
+		putAndGet(orderLinkToItsCreator, toUriList(firstCreatorLink), TEXT_URI_LIST);
+		assertCreatorName(orderLinkToItsCreator, "Frodo");
+
+		putAndGet(orderLinkToItsCreator, toUriList(secondCreatorLink), TEXT_URI_LIST);
+		assertCreatorName(orderLinkToItsCreator, "Pippin");
+	}
+
+	/**
+	 * Negative test scenario, which does try to put two persons at once as the creator of Order. We expect that result
+	 * will contain "send only 1 link" substring.
+	 */
+	@Test // DATAREST-1356
+	public void associateTwoCreatorsToOrderWithSinglePut() throws Exception {
+
+		Link firstCreatorLink = preparePersonResource(new Person("Frodo", "Baggins"));
+		Link secondCreatorLink = preparePersonResource(new Person("Pippin", "Baggins"));
+		Link orderLinkToItsCreator = prepareOrderResource(new Order());
+
+		MockHttpServletResponse response = putOnlyExpect5XXStatus(orderLinkToItsCreator,
+				toUriList(firstCreatorLink, secondCreatorLink), TEXT_URI_LIST);
+		assertThat(response.getContentAsString()).contains("send only 1 link");
+	}
+
 	@Test // DATAREST-219
 	public void manipulatePropertyCollectionRestfullyWithDelete() throws Exception {
 
@@ -673,6 +708,28 @@ public class JpaWebTests extends CommonWebTests {
 		return links;
 	}
 
+	/**
+	 * @return link to creator of order (associative link for given order instance)
+	 */
+	private Link prepareOrderResource(Order order) throws Exception {
+		Link orderLink = client.discoverUnique(LinkRelation.of("orders"));
+
+		MockHttpServletResponse primaryResponse = postAndGet(orderLink, mapper.writeValueAsString(order),
+				MediaType.APPLICATION_JSON);
+		return client.assertHasLinkWithRel("creator", primaryResponse);
+	}
+
+	/**
+	 * @return link to given person (canonical, self, link)
+	 */
+	private Link preparePersonResource(Person person) throws Exception {
+		Link orderLink = client.discoverUnique(LinkRelation.of("people"));
+
+		MockHttpServletResponse primaryResponse = postAndGet(orderLink, mapper.writeValueAsString(person),
+				MediaType.APPLICATION_JSON);
+		return client.assertHasLinkWithRel("self", primaryResponse);
+	}
+
 	/**
 	 * Asserts the {@link Person} resource the given link points to contains siblings with the given names.
 	 *
@@ -689,6 +746,13 @@ public class JpaWebTests extends CommonWebTests {
 		assertThat(persons).contains(siblingNames);
 	}
 
+	private void assertCreatorName(Link orderLinkToItsCreator, String creatorName) throws Exception {
+		String responseBody = client.request(orderLinkToItsCreator).getContentAsString();
+		String personFirstName = JsonPath.read(responseBody, "$.firstName");
+
+		assertThat(personFirstName).isEqualTo(creatorName);
+	}
+
 	private void assertPersonWithNameAndSiblingLink(String name) throws Exception {
 
 		MockHttpServletResponse response = client.request(client.discoverUnique(LinkRelation.of("people")));

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/RepositoryPropertyReferenceController.java

@@ -77,6 +77,7 @@ import org.springframework.web.bind.annotation.RequestMapping;
  * @author Jon Brisbin
  * @author Oliver Gierke
  * @author Greg Turnquist
+ * @author Ľubomír Varga
  */
 @RepositoryRestController
 @SuppressWarnings({ "unchecked" })
@@ -318,7 +319,7 @@ class RepositoryPropertyReferenceController extends AbstractRepositoryRestContro
 									"Cannot PATCH a reference to this singular property since the property type is not a List or a Map.");
 				}
 
-				if (source.getLinks().hasSingleLink()) {
+				if (!source.getLinks().hasSingleLink()) {
 					throw new IllegalArgumentException(
 							"Must send only 1 link to update a property reference that isn't a List or a Map.");
 				}