Return meaningful error for undefined record key

refs #113

packages/skygear-core/lib/record.js

@@ -286,7 +286,11 @@ export default class Record {
       _access: this._access && this._access.toJSON()
     };
     _.each(this.attributeKeys, (key) => {
-      payload[key] = toJSON(this[key]);
+      const value = this[key];
+      if (value === undefined) {
+        throw new Error(`Unsupported undefined value of record key: ${key}`);
+      }
+      payload[key] = toJSON(value);
     });
 
     return payload;

packages/skygear-core/lib/util.js

@@ -20,6 +20,10 @@ import Geolocation from './geolocation';
 import {UnknownValue} from './type';
 
 export function toJSON(v) {
+  if (v === undefined) {
+    throw new Error('toJSON does not support undefined value');
+  }
+
   if (v === null) {
     return null;
   } else if (_.isArray(v)) {

packages/skygear-core/test/record.js

@@ -204,6 +204,16 @@ describe('Extended Record', function () {
     });
   });
 
+  it('serialize with undefined value', function () {
+    const note = new Note({
+      _id: 'note/uid',
+      content: undefined
+    });
+
+    expect(() => note.toJSON())
+      .to.throw('Unsupported undefined value of record key: content');
+  });
+
   it('deserialize from payload with date', function () {
     const note = new Note({
       _id: 'note/uid',

packages/skygear-core/test/util.js

@@ -100,5 +100,10 @@ describe('util', function () {
     ]);
   });
 
+  it('toJSON undefined', function () {
+    expect(() => toJSON(undefined))
+      .to.throw('toJSON does not support undefined value');
+  });
+
 });
 /*eslint-enable no-unused-vars, quote-props */