title: reject_if option is ignored
body: <p>Here's the (simplified) code:</p>
  
  <pre><code>class Biosimilar::AdverseEvent &lt; ActiveRecord::Base
  
    attr_accessibile :adverse_event_med_conds_attributes
  
    has_many :adverse_event_med_conds, 
        :class_name =&gt; 'Biosimilar::AdverseEventMedCond', 
        :dependent =&gt; :destroy
    has_many :med_conds, 
        :class_name =&gt; 'Biosimilar::MedCond', 
        :through =&gt; :adverse_event_med_conds
  
    accepts_nested_attributes_for :adverse_event_med_conds,
        :allow_destroy =&gt; true,
        :reject_if =&gt; proc { |attributes| attributes.any? {|k,v| v.blank?} }
  end    
  </code></pre>
  
  <p>When the form is submitted, the record is created on "adverse_event_med_conds" table even if the user leaves the "med_cond_id" field EMPTY. The reject_if doesn't work!</p>
  
  <p>Any suggestions?</p>