Blob 记录从一个集合到另一个集合(即删除 Join Flatten)
db.posts.find().forEach(function(doc){
if(doc.commentsBlobId){
var commentsBlob = db.comments.findOne({'_id': commentsBlobId });
db.posts.update({_id: doc._id}, {$set:{'comments': commentsBlob }}, false, true);
}
});