duplicate value on unique index error (#4560)

spec/schemas.spec.js

@@ -2387,4 +2387,28 @@ describe('schemas', () => {
       });
     });
   });
+
+  it_exclude_dbs(['postgres'])('cannot update to duplicate value on unique index', (done) => {
+    const index = {
+      code: 1
+    };
+    const obj1 = new Parse.Object('UniqueIndexClass');
+    obj1.set('code', 1);
+    const obj2 = new Parse.Object('UniqueIndexClass');
+    obj2.set('code', 2);
+    const adapter = config.database.adapter;
+    adapter._adaptiveCollection('UniqueIndexClass').then(collection => {
+      return collection._ensureSparseUniqueIndexInBackground(index);
+    }).then(() => {
+      return obj1.save();
+    }).then(() => {
+      return obj2.save();
+    }).then(() => {
+      obj1.set('code', 2);
+      return obj1.save();
+    }).then(done.fail).catch((error) => {
+      expect(error.code).toEqual(Parse.Error.DUPLICATE_VALUE);
+      done();
+    });
+  });
 });

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -421,7 +421,13 @@ export class MongoStorageAdapter implements StorageAdapter {
     const mongoWhere = transformWhere(className, query, schema);
     return this._adaptiveCollection(className)
       .then(collection => collection._mongoCollection.findAndModify(mongoWhere, [], mongoUpdate, { new: true }))
-      .then(result => mongoObjectToParseObject(className, result.value, schema));
+      .then(result => mongoObjectToParseObject(className, result.value, schema))
+      .catch(error => {
+        if (error.code === 11000) {
+          throw new Parse.Error(Parse.Error.DUPLICATE_VALUE, 'A duplicate value for a field with unique values was provided');
+        }
+        throw error;
+      });
   }
 
   // Hopefully we can get rid of this. It's only used for config and hooks.