title: Rails admin hide belongs_to field in has_many nested form
body: <p>I have two Models</p>
  
  <pre><code>class Entity &lt; ActiveRecord::Base
    # Associations
    has_many :contacts
    accepts_nested_attributes_for :contacts, :allow_destroy =&gt; true
  end
  
  class Contact &lt; ActiveRecord::Base
    # Associations
    belongs_to :entity
  end
  </code></pre>
  
  <p>Now in rails admin I am getting below options.</p>
  
  <p><strong>Add new Contact Form</strong></p>
  
  <p><img src="https://i.stack.imgur.com/dTve9.png" alt="enter image description here"></p>
  
  <hr>
  
  <p><strong>Add new Entity Form</strong></p>
  
  <p><img src="https://i.stack.imgur.com/vthap.png" alt="enter image description here"></p>
  
  <p>I need to hide Entity field in contact form , while adding new entity.</p>
  
  <p>Any help will be useful.</p>