Allowing relationship links to get serialized for null relationship object

src/main/java/com/github/jasminb/jsonapi/ResourceConverter.java

@@ -842,19 +842,36 @@ private ObjectNode getDataNode(Object object, Map<String, ObjectNode> includedCo
 
 				removeField(attributesNode, relationshipField);
 
-				if (relationshipObject != null) {
+				Relationship relationship = configuration.getFieldRelationship(relationshipField);
 
-					Relationship relationship = configuration.getFieldRelationship(relationshipField);
+				// In case serialisation is disabled for a given relationship, skip it
+				if (!relationship.serialise()) {
+					continue;
+				}
 
-					// In case serialisation is disabled for a given relationship, skip it
-					if (!relationship.serialise()) {
-						continue;
-					}
+				// Serialize relationship object
+				ObjectNode relationshipDataNode = objectMapper.createObjectNode();
+
+				String relationshipName = relationship.value();
+
+				// Allow relationships node to have relationship data node even if relationship object is null
+				relationshipsNode.set(relationshipName, relationshipDataNode);
+
+				// Serialize relationship links
+				JsonNode relationshipLinks = getRelationshipLinks(object, relationship, selfHref, settings);
+
+				if (relationshipLinks != null) {
+					// Allow relationship data node to have links even if relationship object is null
+					relationshipDataNode.set(LINKS, relationshipLinks);
+
+					// Remove link object from serialized JSON
+					Field refField = configuration
+							.getRelationshipLinksField(object.getClass(), relationshipName);
 
-					String relationshipName = relationship.value();
+					removeField(attributesNode, refField);
+				}
 
-					ObjectNode relationshipDataNode = objectMapper.createObjectNode();
-					relationshipsNode.set(relationshipName, relationshipDataNode);
+				if (relationshipObject != null) {
 
 					// Serialize relationship meta
 					JsonNode relationshipMeta = getRelationshipMeta(object, relationshipName, settings);
@@ -868,19 +885,6 @@ private ObjectNode getDataNode(Object object, Map<String, ObjectNode> includedCo
 						removeField(attributesNode, refField);
 					}
 
-					// Serialize relationship links
-					JsonNode relationshipLinks = getRelationshipLinks(object, relationship, selfHref, settings);
-
-					if (relationshipLinks != null) {
-						relationshipDataNode.set(LINKS, relationshipLinks);
-
-						// Remove link object from serialized JSON
-						Field refField = configuration
-								.getRelationshipLinksField(object.getClass(), relationshipName);
-
-						removeField(attributesNode, refField);
-					}
-
 					boolean shouldSerializeData = configuration.getFieldRelationship(relationshipField).serialiseData();
 					if (shouldSerializeData) {
 						if (relationshipObject instanceof Collection) {

src/test/java/com/github/jasminb/jsonapi/ResourceConverterTest.java

@@ -122,12 +122,19 @@ public void testReadWithIncludedSection() throws IOException {
 	}
 
 	@Test
-	public void testWriteCollection() throws IOException, IllegalAccessException {
+	public void testWriteCollection() throws DocumentSerializationException, IOException {
 		InputStream usersRequest = IOUtils.getResource("users.json");
 
 		JSONAPIDocument<List<User>> usersDocument = converter.readDocumentCollection(usersRequest, User.class);
 		List<User> users = usersDocument.get();
-		byte[] convertedData = converter.writeObjectCollection(users);
+
+		assertNotNull(users);
+		assertEquals(2, users.size());
+
+		// Make sure that relationship object i.e. statuses is null
+		assertNull(users.get(0).getStatuses());
+
+		byte[] convertedData = converter.writeDocumentCollection(usersDocument);
 
 		assertNotNull(convertedData);
 		assertFalse(convertedData.length == 0);
@@ -140,14 +147,73 @@ public void testWriteCollection() throws IOException, IllegalAccessException {
 		ObjectMapper mapper = new ObjectMapper();
 		try {
 			JsonNode node1 = mapper.readTree(IOUtils.getResource("users.json"));
+
+			// Make sure relationship node always get serialized even if relationship object i.e. statuses is null
+			JsonNode user1Relationships = node1.get("data").get(0).get("relationships");
+			assertNotNull(user1Relationships.get("statuses"));
+
+			JsonNode user2Relationships = node1.get("data").get(1).get("relationships");
+			assertNotNull(user2Relationships.get("statuses"));
+
 			JsonNode node2 = mapper.readTree(convertedData);
-			assertEquals(node1, node2);
+
+			// Make sure relationship node must always contains one of either meta, link or data node
+			user1Relationships = node2.get("data").get(0).get("relationships");
+			assertNotNull(user1Relationships.get("statuses"));
+			assertNotNull(user1Relationships.get("statuses").get("links"));
+
+			user2Relationships = node2.get("data").get(1).get("relationships");
+			assertNotNull(user2Relationships.get("statuses"));
+			assertNotNull(user2Relationships.get("statuses").get("links"));
+
+			assertNotEquals(node1, node2);
 		} catch (IOException e) {
 			throw new RuntimeException("Unable to read json, make sure is correct", e);
 		}
 	}
 
 	@Test
+	public void testLinksForNonIncludedEmptyToManyRelationship() throws IOException, IllegalAccessException {
+		InputStream apiResponse = IOUtils.getResource("articles-with-non-included-empty-to-many-relationship.json");
+
+		ObjectMapper articlesMapper = new ObjectMapper();
+		articlesMapper.setPropertyNamingStrategy(PropertyNamingStrategy.KEBAB_CASE);
+
+		ResourceConverter articlesConverter = new ResourceConverter(articlesMapper, Article.class, Author.class,
+				Comment.class);
+
+		JSONAPIDocument<List<Article>> articlesDocument = articlesConverter.readDocumentCollection(apiResponse, Article.class);
+		List<Article> articles = articlesDocument.get();
+
+		assertNotNull(articles);
+		assertEquals(1, articles.size());
+
+		Article article = articles.get(0);
+
+		assertNull(article.getComments());
+
+		assertNull(article.getCommentRelationshipLinks());
+
+		byte[] convertedData = converter.writeObjectCollection(articles);
+		assertNotNull(convertedData);
+		assertNotEquals(0, convertedData.length);
+
+		JSONAPIDocument<List<Article>> convertedDocument = converter.readDocumentCollection(new ByteArrayInputStream(convertedData), Article.class);
+		List<Article> convertedArticles = convertedDocument.get();
+		assertNotNull(convertedArticles);
+
+		Article convertedArticle = convertedArticles.get(0);
+
+		assertNull(convertedArticle.getComments());
+
+		// Make sure Relationship links are getting serialized even if relationship object i.e. comments is null
+		assertNotNull(convertedArticle.getCommentRelationshipLinks());
+		assertEquals("https://api.example.com/articles/1/relationships/comments", convertedArticle.getCommentRelationshipLinks().getSelf().toString());
+		assertEquals("https://api.example.com/articles/1/comments", convertedArticle.getCommentRelationshipLinks().getRelated().toString());
+
+	}
+
+    @Test
 	public void testReadWithMetaAndLinksSection() throws IOException {
 		InputStream apiResponse = IOUtils.getResource("user-with-meta.json");
 

src/test/java/com/github/jasminb/jsonapi/SerializationTest.java

@@ -187,6 +187,7 @@ public void testSerializeWithoutId() throws DocumentSerializationException {
 		User user = new User();
 		user.setName("Name");
 
+		converter.disableSerializationOption(SerializationFeature.INCLUDE_LINKS);
 		byte [] data = converter.writeDocument(new JSONAPIDocument<>(user));
 
 		Assert.assertTrue(new String(data).contains(user.getName()));

src/test/java/com/github/jasminb/jsonapi/models/Article.java

@@ -13,7 +13,7 @@
 import java.util.Collections;
 import java.util.List;
 
-@Type("articles")
+@Type(value = "articles", path = "/articles/{id}")
 @JsonIdentityInfo(generator = ObjectIdGenerators.StringIdGenerator.class, property = "id")
 public class Article {
 	@Id
@@ -24,9 +24,12 @@ public class Article {
 	@Relationship(value = "author", resolve = true, relType = RelType.RELATED)
 	private Author author;
 
-	@Relationship(value = "comments", resolve = true)
+	@Relationship(value = "comments", resolve = true, path = "relationships/comments", relatedPath = "comments")
 	private List<Comment> comments;
 
+    @RelationshipLinks(value = "comments")
+    private Links commentRelationshipLinks;
+
 	@Relationship(value = "users", serialiseData = false)
 	private List<User> users;
 
@@ -84,4 +87,12 @@ public Links getUserRelationshipLinks() {
 	public void setUserRelationshipLinks(Links userRelationshipLinks) {
 		this.userRelationshipLinks = userRelationshipLinks;
 	}
+
+    public Links getCommentRelationshipLinks() {
+        return commentRelationshipLinks;
+    }
+
+    public void setCommentRelationshipLinks(Links commentRelationshipLinks) {
+        this.commentRelationshipLinks = commentRelationshipLinks;
+    }
 }

src/test/java/com/github/jasminb/jsonapi/models/User.java

@@ -10,7 +10,7 @@
 
 import java.util.List;
 
-@Type("users")
+@Type(value = "users", path = "/users/{id}")
 @JsonIdentityInfo(generator = ObjectIdGenerators.StringIdGenerator.class, property = "id")
 public class User {
 
@@ -26,7 +26,7 @@ public String getToken() {
 	public String id;
 	public String name;
 
-	@Relationship("statuses")
+	@Relationship(value = "statuses", path = "/relationship/statuses")
 	private List<Status> statuses;
 
 	@Meta

src/test/resources/articles-with-non-included-empty-to-many-relationship.json

@@ -0,0 +1,39 @@
+{
+  "data": [{
+    "type": "articles",
+    "id": "1",
+    "attributes": {
+      "title": "JSON API paints my bikeshed!"
+    },
+    "links": {
+      "self": "http://example.com/articles/1"
+    },
+    "relationships": {
+      "author": {
+        "links": {
+          "self": "http://example.com/articles/1/relationships/author",
+          "related": "http://example.com/articles/1/author"
+        },
+        "data": { "type": "people", "id": "9" }
+      },
+      "users": {
+        "links": {
+          "self": "http://example.com/articles/1/relationships/users",
+          "related": "http://example.com/articles/1/users"
+        }
+      }
+    }
+  }],
+  "included": [{
+    "type": "people",
+    "id": "9",
+    "attributes": {
+      "first-name": "Dan",
+      "last-name": "Gebhardt",
+      "twitter": "dgeb"
+    },
+    "links": {
+      "self": "http://example.com/people/9"
+    }
+  }]
+}

src/test/resources/users.json

@@ -5,13 +5,19 @@
       "id": "1",
       "attributes": {
         "name": "liz"
+      },
+      "relationships": {
+        "statuses": {}
       }
     },
     {
       "type": "users",
       "id": "2",
       "attributes": {
         "name": "john"
+      },
+      "relationships": {
+        "statuses": {}
       }
     }
   ]