title: Nested form in activeadmin not saving updates
body: <p>I have a nested form in ActiveAdmin for these models (a :class_section has_many :class_dates): </p>
  
  <pre><code>class ClassDate &lt; ActiveRecord::Base
    belongs_to :class_section
    validates :start_time, :presence =&gt; true
    validates :end_time, :presence =&gt; true
  
  end
  </code></pre>
  
  <p>and</p>
  
  <pre><code>class ClassSection &lt; ActiveRecord::Base
    belongs_to :class_course
    has_many :class_dates
    belongs_to :location
  
    accepts_nested_attributes_for :class_dates
  end
  </code></pre>
  
  <p>Everything seems to be in the right place when I look at the form. However, when I update a class_date, it doesn't save.</p>
  
  <pre><code>ActiveAdmin.register ClassSection do
  
    permit_params :max_students, :min_students, :info, :class_course_id, :location_id
  
    form do |f|
      f.inputs "Details" do
        f.input :class_course, member_label: :id_num
        f.input :min_students, label: "Minunum Students"
        f.input :max_students, label: "Maxiumum Students"
        f.input :location
      end
      f.inputs do
        f.input :info, label: "Additional Information"
      end
      f.inputs "Dates" do
        f.has_many :class_dates, heading: false do |cd|
          cd.input :start_time, :as =&gt; :datetime_picker
          cd.input :end_time, :as =&gt; :datetime_picker
        end
      end
      f.actions
    end
  
    index do
      column :class_course
      column :location
      default_actions
    end
  
    filter :class_course
    filter :location
  
    show do |cs|
      attributes_table do
        row :class_course do
          cs.class_course.id_num + " - " + cs.class_course.name
        end
        row :location
        row :min_students, label: "Minunum Students"
        row :max_students, label: "Maxiumum Students"
        row :info, label: "Additional Information"
      end
  
      panel "Dates" do
        attributes_table_for class_section.class_dates do
          rows :start_time, :end_time
        end
      end
      active_admin_comments
    end 
  
  end
  </code></pre>
  
  <p>Here is the ActiveAdmin file for ClassDates:</p>
  
  <pre><code>ActiveAdmin.register ClassDate, as: "Dates" do
  
    permit_params :start_time, :end_time, :class_section_id
  
    belongs_to :class_section
  
  end
  </code></pre>
  
  <p>Can you see a reason why it's not saving properly?</p>
  
  <p><strong>UPDATE:</strong> I added the following code to the AA and it seems to work now:</p>
  
  <pre><code>controller do
    def permitted_params
      params.permit!
    end
  end
  </code></pre>
  
  <p>Let me know if there is a better solution. Thanks.</p>
  
  <p><strong>UPDATE 2:</strong> There is one lingering problem however. I am unable to delete ClassDates using this form.</p>