mongoodb 操作数组有毒！

我定义了一个数组结构 ,如下：

imgUrls:	 {	type: Array	} 

我在更新的时候使用 save( ) 方法，之后就疯狂的给我保错：

{ MongoError: An empty update path is not valid.
    at Function.MongoError.create (/Users/yangyss/Desktop/Cool/git/react-koa/node_modules/mongodb-core/lib/error.js:31:11)
    at toError (/Users/yangyss/Desktop/Cool/git/react-koa/node_modules/mongodb/lib/utils.js:114:22)
    at /Users/yangyss/Desktop/Cool/git/react-koa/node_modules/mongodb/lib/collection.js:1049:67
    at /Users/yangyss/Desktop/Cool/git/react-koa/node_modules/mongodb-core/lib/connection/pool.js:455:18
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)
  name: 'MongoError',
  message: 'An empty update path is not valid.',
  driver: true,
  index: 0,
  code: 56,
  errmsg: 'An empty update path is not valid.' }

想问一下大家是怎么操作数组的，我想随意替换数组，比如： [1,2,3] 更新成 [4,5,6]