title: rails_admin - has_many association - default value
body: <p>My Page model look like this:</p>
  
  <pre><code>class Page &lt; ActiveRecord::Base
    has_many :blocks
  
    accepts_nested_attributes_for :blocks, allow_destroy: true
  
    rails_admin do
      edit do
        fields :title, :slug, :blocks
      end
    end
  end
  </code></pre>
  
  <p>My Block model look like this:</p>
  
  <pre><code>class Block &lt; ActiveRecord::Base
    belongs_to :page
  
    rails_admin do
      edit do
        field :title
        field :body, :ck_editor
      end
    end
  end
  </code></pre>
  
  <p>I needed workflow like this:
  As an admin I click create page and I should see opened new block section with prefield title.</p>
  
  <p><img src="https://i.stack.imgur.com/1Fmvw.jpg" alt="enter image description here"></p>
  
  <p>How can I create this scenario?</p>