预先占位然后删除
预先占位在一个额外的字段，然后取消
comments = [ {name : "Name Surname", date : "1970.01.01", text : "this is a placeholder"}, {name : "Name Surname", date : "1970.01.01", text : "this is a placeholder"} ]db.posts.insert({ comments: [], padding: 'lorep ipsum ... x N'})db.posts.update(query, {$push: {comments: comment_data}, $unset: padding})?