title: Mongoid has_many / belongs_to has 'undefined method' error
body: <p>I want to use has_many/belongs_to in my mongoid models.</p>
  
  <p>I already use that in other models and it works well, but in the codes below occurs </p>
  
  <pre><code>undefined method `options' for class `Product' error. 
  </code></pre>
  
  <p>Models :</p>
  
  <pre><code>class Product
    include Mongoid::Document
    include Mongoid::Timestamps
  
    has_many :options
  
    field :name, type: String
    ...
  end
  
  class Option
    include Mongoid::Document
    include Mongoid::Timestamps
  
    belongs_to :product
  
    field :name, type: String
  end
  </code></pre>
  
  <p>Controller :</p>
  
  <pre><code>class ProductsController &lt; ApplicationController
    def index
      @products = Product.order(id: :desc)
    end
  end
  </code></pre>
  
  <p>How can I fix the error?</p>