allow relationships to have an alternative key

closes #12

Author: danivek

Diff for: README.md

@@ -35,6 +35,7 @@ Serializer.register(type, options);
 - **relationships** (optional): An object defining some relationships
     - relationship: The property in data to use as a relationship
         - **type**: The type to use for serializing the relationship (type need to be register)
+        - **alternativeKey** (optional): An alternative key to use if relationship key not exist (example: 'author_id' as an alternative key for 'author' relationship). See [issue #12](https://github.com/danivek/json-api-serializer/issues/12).
         - **schema** (optional): A custom schema for serializing the relationship. If no schema define, it use the default one.
         - **links** (optional): An *object* or a *function* that describes the links for the relationship. (If it is an object values can be string or function).
 - **convertCase** (optional): Case conversion for outputted data. Value can be : `kebab-case`, `snake_case`, `camelCase`

Diff for: lib/JSONAPISerializer.js

@@ -37,6 +37,7 @@ module.exports = class JSONAPISerializer {
       links: joi.alternatives([joi.func(), joi.object()]).default({}),
       relationships: joi.object().pattern(/.+/, joi.object({
         type: joi.string().required(),
+        alternativeKey: joi.string(),
         schema: joi.string(),
         links: joi.alternatives([joi.func(), joi.object()]).default({}),
       })).default({}),
@@ -183,7 +184,16 @@ module.exports = class JSONAPISerializer {
       data = _.pick(data, options.whitelist);
     }
 
-    let serializedAttributes = _.pick(data, _.difference(Object.keys(data), _.concat([options.id], Object.keys(options.relationships), options.blacklist)));
+    // Support alternativeKey options for relationships
+    const alternativeKeys = [];
+    _.forOwn(options.relationships, (rOptions) => {
+      if (rOptions.alternativeKey) {
+        alternativeKeys.push(rOptions.alternativeKey);
+      }
+    });
+
+    // Remove unwanted keys (id, blacklist, relationships, alternativeKeys)
+    let serializedAttributes = _.pick(data, _.difference(Object.keys(data), _.concat([options.id], Object.keys(options.relationships), alternativeKeys, options.blacklist)));
 
     if (options.convertCase) {
       serializedAttributes = this._convertCase(serializedAttributes, options.convertCase);
@@ -209,9 +219,15 @@ module.exports = class JSONAPISerializer {
     _.forOwn(options.relationships, (rOptions, relationship) => {
       const schema = rOptions.schema || 'default';
 
+      // Support alternativeKey options for relationships
+      let relationshipKey = relationship;
+      if (!data[relationship] && rOptions.alternativeKey) {
+        relationshipKey = rOptions.alternativeKey;
+      }
+
       const serializeRelationship = {
         links: this.processOptionsValues(data, rOptions.links),
-        data: this.serializeRelationship(rOptions.type, data[relationship], this.schemas[options.relationships[relationship].type][schema], included),
+        data: this.serializeRelationship(rOptions.type, data[relationshipKey], this.schemas[options.relationships[relationship].type][schema], included),
       };
 
       relationship = (options.convertCase) ? this._convertCase(relationship, options.convertCase) : relationship;

Diff for: test/unit/JSONAPISerializer.test.js

@@ -333,6 +333,32 @@ describe('JSONAPISerializer', function() {
       expect(serializedRelationships).to.have.property('article-authors');
       done();
     });
+
+    it('should return relationships with alternativeKey option if relationship key not exist', function(done) {
+      const included = [];
+      const Serializer = new JSONAPISerializer();
+
+      Serializer.register('article', {
+        relationships: {
+          author: {
+            type: 'people',
+            alternativeKey: 'author_id'
+          }
+        }
+      });
+      Serializer.register('people');
+
+      const serializedRelationships = Serializer.serializeRelationships({
+        id: '1',
+        author_id: '1'
+      }, Serializer.schemas.article.default, included);
+      expect(serializedRelationships).to.have.property('author');
+      expect(serializedRelationships.author).to.have.property('data');
+      expect(serializedRelationships.author.data).to.have.property('type').to.eql('people');
+      expect(serializedRelationships.author.data).to.have.property('id').to.be.a('string').to.eql('1');
+      expect(serializedRelationships.author).to.have.property('links').to.be.undefined;
+      done();
+    });
   });
 
   describe('serializeAttributes', function() {
@@ -455,6 +481,35 @@ describe('JSONAPISerializer', function() {
       expect(serializedAttributes.address).to.have.property('zipCode');
       done();
     });
+
+    it('should not return alternativeKey option on relationships', function(done) {
+      const Serializer = new JSONAPISerializer();
+      Serializer.register('people', {});
+      Serializer.register('article', {
+        relationships: {
+          author: {
+            type: 'people',
+            alternativeKey: 'author_id'
+          }
+        }
+      });
+
+      const data = {
+        id: '1',
+        title: 'Nice article',
+        author_id: '1',
+        author: {
+          id: '1'
+        },
+      };
+
+      const serializedAttributes = Serializer.serializeAttributes(data, Serializer.schemas.article.default);
+      expect(serializedAttributes).to.not.have.property('id'); // No identifier
+      expect(serializedAttributes).to.not.have.property('author_id'); // No relationship alternativeKey
+      expect(serializedAttributes).to.not.have.property('author'); // No relationship key
+      expect(serializedAttributes).to.have.property('title');
+      done();
+    });
   });
 
   describe('serializeIncluded', function() {