Angular for循环中的承诺

我有一个像这样的对象数组:

var  myArray = [{post:{message:"hi",user:"joe"}},{post:{message:"how are you",user:"bob"}}];

其中myArray[0].post.user = "joe"和myArray[1].post.user = "bob"

我想添加另一个键(即comment)到post动态基于异步调用使用$resource，而通过数组循环。

远程Comments服务运行良好，看起来像这样:

.factory('Comments',function($resource, myUrl) {
    var comments = $resource(myUrl + 'api/comments/:postId', {postId: '@id'});
    return comments;
})

in my controller

var  myArray = [{post:{message:"hi",user:"joe"}},{post:{message:"how are you",user:"bob"}}];
var messageCount = myArray.length;
var post_id = 1;   //comes from elsewhere
for (var c = 0; c < messageCount; c++) {
    Comments.get({postId:post_id}).$promise.then(function(comments) {
      console.log(comments);     //  <<---- objects appear to be fine in the console log
      myArray[c].post.comments = comments;  // <---- throws undefined error 
    });
 }

为什么myArray.post.comments = comments抛出错误Cannot set property 'user' of undefined ?