title: Mongoid persistance issue
body: <p>I'm trying to assign an attribute in my Rails app, however, the assigned data disappears when I re-query the database. Here are my models:</p>
  
  <pre><code>class Scent
    include Mongoid::Document
  
    field :scentid, type: Integer
    field :name
    field :price
    field :category
    field :description
    field :available, type: Boolean
  
    belongs_to :order
    embedded_in :cartitem
  
    attr_accessible :name, :price, :category, :available
  end
  
  class Cartitem
    include Mongoid::Document  
  
    field :quantity, type: Integer
  
    embeds_one :scent
    embedded_in :cart
  end
  </code></pre>
  
  <p>Here is the query I am running:</p>
  
  <pre><code>1.9.3p194 :001 &gt; User.first.cart.cartitems.first.scent
  =&gt; nil
  1.9.3p194 :060 &gt; User.first.cart.cartitems.first.scent = Scent.first
  =&gt; #&lt;Scent _id: 50381e2ec8bafa1710000001, _type: nil, scentid: 1, name: "gold", price"99.99", category: "fresh", description: nil, available: true, order_id: nil&gt;
  1.9.3p194 :061 &gt; User.first.cart.cartitems.first.scent
  =&gt; nil 
  </code></pre>
  
  <p>Any ideas on why this is happening? Thanks for the help!</p>
  
  <p><strong>EDIT:</strong> here are my User and Cart models:</p>
  
  <pre><code>  field :firstname
    field :lastname
    field :email
    field :password
    field :password_confirmation
    field :password_digest
    field :stripeid
    field :remember_token
  
    has_many :orders
    embeds_many :address
    embeds_one :cart
  
    attr_accessible :firstname, :lastname, :email, :password, :password_confirmation
    has_secure_password
    before_save { |user| user.email = email.downcase }
    before_save :create_remember_token
  
    validates :firstname, presence: true, length: {maximum: 50}
    validates :lastname, presence: true, length: {maximum: 50}
    VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
    validates :email, presence: true, format: {with: VALID_EMAIL_REGEX}, uniqueness:{case_sensitive: false}
    validates :password, presence: true, length: {minimum: 6}
    validates :password_confirmation, presence: true
  
    index({email:1}, {unique: true, name: 'email_index'})
  
    private
      def create_remember_token
      self.remember_token = SecureRandom.urlsafe_base64
    end
  end
  
  class Cart
    include Mongoid::Document  
  
    embedded_in :user
    embeds_many :cartitems
  
  end
  </code></pre>