title: How to prefill ActiveAdmin form value from another model?
body: <p>In the following code, how does one populate a prefilled form field with a value from another model into the ActiveAdmin form:</p>
  
  <pre><code>ActiveAdmin.register Person do
  
    index do
      column :forename
      column :surname
      column :code do |p|
        MyCode.find_by_person_id(p.id).code
      end
      column :updated_at
      default_actions
    end
  
    filter :forename
    filter :surname
  
    form do |f|
      f.inputs "Person" do
        f.input :forename
        f.input :surname
        # How to get a value here, e.g. MyCode.find_by_person_id(p.id).code as above
        #f.input :code, :input_html =&gt; { :value =&gt; value??? }, as: :hidden
      end
      f.actions
    end
  end
  </code></pre>