Improve validation error message when field names conflict (#363)

* Improve validation error message when field names conflict

* Remove extra hint and add unit test

package.json

@@ -61,6 +61,7 @@
     "babel-plugin-transform-runtime": "6.6.0",
     "babel-preset-es2015": "6.6.0",
     "chai": "3.5.0",
+    "chai-string": "1.2.0",
     "chai-subset": "1.2.2",
     "coveralls": "2.11.9",
     "eslint": "2.7.0",

src/validation/__tests__/OverlappingFieldsCanBeMerged-test.js

@@ -7,6 +7,7 @@
  *  of patent rights can be found in the PATENTS file in the same directory.
  */
 
+import { expect } from 'chai';
 import { describe, it } from 'mocha';
 import {
   expectPassesRule,
@@ -28,7 +29,10 @@ import {
   GraphQLString,
   GraphQLID,
 } from '../../type';
+import chai from 'chai';
+import chaiString from 'chai-string';
 
+chai.use(chaiString);
 
 describe('Validate: Overlapping fields can be merged', () => {
 
@@ -753,6 +757,15 @@ describe('Validate: Overlapping fields can be merged', () => {
       `);
     });
 
+    it('error message contains hint for alias conflict', () => {
+      // The error template should end with a hint for the user to try using
+      // different aliases.
+      const error = fieldsConflictMessage('x', 'a and b are different fields');
+      const hint = 'Use different aliases on the fields to fetch both ' +
+                   'if this was intentional.';
+      expect(error).to.endsWith(hint);
+    });
+
   });
 
 });

src/validation/rules/OverlappingFieldsCanBeMerged.js

@@ -39,7 +39,9 @@ export function fieldsConflictMessage(
   responseName: string,
   reason: ConflictReasonMessage
 ): string {
-  return `Fields "${responseName}" conflict because ${reasonMessage(reason)}.`;
+  return `Fields "${responseName}" conflict because ${reasonMessage(reason)}` +
+    '. Use different aliases on the fields to fetch both if this was ' +
+    'intentional.';
 }
 
 function reasonMessage(reason: ConflictReasonMessage): string {