Rails create and update array in database

So I have a service object that submits up votes and voted users into a database:

Here's the posts model:

class Post < ActiveRecord::Base
  attr_accessible :comment_count, :downvote, :id, :text, :title, :upvote, :url, :user_id, :users_voted_up_by, :users_voted_down_by

  serialize :users_voted_up_by, Array
  serialize :users_voted_down_by, Array

  belongs_to :user
end

Here's the User model:

class User < ActiveRecord::Base
  attr_accessible :email, :password, :password_confirmation, :username, :good_karma, :bad_karma, :posts_voted_up_on, :posts_voted_down_on

  serialize :posts_voted_up_on, Array
  serialize :posts_voted_down_on, Array

  has_many :posts

  attr_accessor :password
  before_save :encrypt_password

  validates_confirmation_of :password
  validates_presence_of :password, :on => :create
  validates_presence_of :email
  validates_uniqueness_of :email
  validates_presence_of :username
  validates_uniqueness_of :username

  def self.authenticate(email, password)
    user = find_by_email(email)
    if user && user.password_hash == BCrypt::Engine.hash_secret(password, user.password_salt)
      user
    else
      nil
    end
  end

  def encrypt_password
    if password.present?
      self.password_salt = BCrypt::Engine.generate_salt
      self.password_hash = BCrypt::Engine.hash_secret(password, password_salt)
    end
  end
end

Now here's my Voter class that controls up votes and down votes.

class Voter
  def initialize(post, user)
    @post = post
    @user = user
  end

  def upvote
    return false unless @post.users_voted_up_by
    @post.upvote += 1
    @post.users_voted_up_by << @user.username
    @user.good_karma += 1
    @post.save && @user.save
  end

  def downvote
    return false unless @post.users_voted_down_by
    @post.upvote += 1
    @post.users_voted_down_by << @user.username
    @user.bad_karma += 1
    @post.save && @user.save
  end
end

It adds and retrevies the first one fine:

User1

But when I perform another "upvote" with a different user, instead of adding it to the array it just adds it to the string like this:

User1User2

Am I not using the line correctly?

@post.users_voted_up_by << @user.username