$ elemMatch（projection）返回选定的数组元素

Question

var ids = ['50478c35889a450000000001',  '50478c35889a450000000002'];
var posterIdArray = ['50478c35889a450000000234',  '50478c35889a450000000234']; 
Post.find({_id: {$in: posterIdArray})
  .select({ comments: { $elemMatch: {$in: ids }}})
  .exec(function (err, doc) {
     console.log(doc.comments); //  ['50478c35889a450000000001']    

});

//注意：只返回一个评论ID //我需要找到一条帖子，其中一些评论受到id和过滤器的限制