Q:

Delete object from array in mongodb

I have simple application on rails with Mongodb(Mondoid mapper). Scaffold posts, and I trying to implement rating functional. Post has next structure:
> class Post
  include Mongoid::Document
  include Mongoid::Timestamps

  field :title, type: String
  field :description, type: String
  field :rank, type: Integer, default: "0"
  field :voter_up, type: Array, default: []
  field :voter_down, type: Array, default: []  

As you can see there are two field voter_up, _down. It's to add users, depending on how they voted. self.voter_up << user.id, or self.voter_down << user.id. In database it looks like:
"voter_up" : [
    ObjectId("4ee08b6e405f3d0a29000005"),
    ObjectId("4ee0aaff405f3d0a2900003a")
]

And if user changed his mind and wants to vote contrary. For example in first time user voted UP. And then voted DOWN. In this situation I need to delete this user from array voter_up and add to voter_down. From rails console it works array.delete(object), but if I try that self.voter_up.delete(user.id) in model function - nothing happens. Why?

A:

I am afraid that both of your console and modal would work. Because the delete here you are using is a ruby array method not the mongoids. So You need to call save after delete to push the changed array to server. 
     self.voter_up.delete(user.id)
     self.save

