Empty relationships returned as data: null

closes #41

lib/JSONAPISerializer.js

@@ -576,12 +576,19 @@ module.exports = class JSONAPISerializer {
         throw new Error(`No schema "${rOptions.schema}" registered for type "${rOptions.type}" on "${relationship}" relationship`);
       }
 
-      const serializeRelationship = {
+      let serializeRelationship = {
         links: this.processOptionsValues(data, extraData, rOptions.links),
         meta: this.processOptionsValues(data, extraData, rOptions.meta),
         data: this.serializeRelationship(rOptions.type, data[relationshipKey], this.schemas[rOptions.type][schema], included, extraData),
       };
 
+      // Avoid empty relationship object
+      if (serializeRelationship.data === undefined && serializeRelationship.links === undefined && serializeRelationship.meta === undefined) {
+        serializeRelationship = {
+          data: null,
+        };
+      }
+
       relationship = (options.convertCase) ? this._convertCase(relationship, options.convertCase) : relationship;
 
       _.set(serializedRelationships, relationship, serializeRelationship);

test/unit/JSONAPISerializer.test.js

@@ -407,6 +407,22 @@ describe('JSONAPISerializer', function() {
       done();
     });
 
+    it('should return at least data null if no links, data, or meta are deduce', function(done) {
+      const included = [];
+      const serializedRelationships = Serializer.serializeRelationships({
+        id: '1',
+      }, Serializer.schemas.articles.default, included);
+      expect(serializedRelationships).to.eql({
+        author: {
+          data: null
+        },
+        comments: {
+          data: null
+        }
+      })
+      done();
+    });
+
     it('should return relationships for author and comments', function(done) {
       const included = [];
       const serializedRelationships = Serializer.serializeRelationships({