title: Nested attributes problem in rails 3 with mongoid (nested objects aren`t saved)
body: <p>So I have a simple application as in <a href="http://railscasts.com/episodes/196-nested-model-form-part-1" rel="nofollow">RailsCast about nested forms.</a> and a problem is that when I submit the form (with survey and questions) <strong>the questions are not saved</strong>. </p>
  
  <p>My models (Survey, that has many Questions):</p>
  
  <pre><code>class Survey
    include Mongoid::Document
    field :name
    has_many :questions, :dependent =&gt; :destroy
    accepts_nested_attributes_for :questions, :allow_destroy =&gt; true
  end
  
  class Question
    include Mongoid::Document
    field :content
    belongs_to :survey
  end
  </code></pre>
  
  <p>and Survey Controller:</p>
  
  <pre><code>def new
      @survey = Survey.new
      3.times {@survey.questions.build}
  ....
  </code></pre>
  
  <p>and a view:</p>
  
  <pre><code>&lt;%= form_for(@survey) do |f| %&gt;
      &lt;%= f.fields_for :questions do |builder| %&gt;
      &lt;%= builder.label :content, "Question" %&gt;&lt;br /&gt;
      &lt;%= builder.text_area :content, :rows =&gt; 3 %&gt;&lt;br /&gt;
      &lt;%= builder.check_box :_destroy %&gt;
      &lt;%= builder.label :_destroy, "Remove Question" %&gt;
      &lt;% end %&gt;
  ...
  </code></pre>
  
  <p>In my log I have:</p>
  
  <pre><code>    Started POST "/surveys" for 127.0.0.1 at 2011-05-24 13:26:51 +0400
    Processing by SurveysController#create as HTML
    Parameters: {"utf8"=&gt;"G£ô", "authenticity_token"=&gt;"tX0FfMiLbh1BwjuY4CuvAKt2UpTraY3vmdo58ocBnos=", "survey"=&gt;{"name"=&gt;"
  Rails", "questions_attributes"=&gt;{"0"=&gt;{"content"=&gt;"Are you fond of Rails?", "_destroy"=&gt;"0"}, "1"=&gt;{"content"=&gt;"Rails is
   the best, ha?", "_destroy"=&gt;"0"}, "2"=&gt;{"content"=&gt;"How many railscasts have you watched?", "_destroy"=&gt;"0"}}}, "commit
  "=&gt;"Create Survey"}
  MONGODB nested_attributes_development['surveys'].insert([{"name"=&gt;"Rails", "_id"=&gt;BSON::ObjectId('4ddb79dba5372914380000
  69')}])
  Redirected to http://localhost:3000/surveys/4ddb79dba537291438000069
  </code></pre>