title: rails_admin - customizing searchable filters based on object_label_method
body: <p>I'm building some kind of a grading system for practice. In rails_admin.rb I have:</p>
  
  <pre><code>config.model Student do
      object_label_method do
        :student_code
      end
  end
  
  config.model Grade do
      list do
        field :student do
          searchable :student_code
        end
        field :course
        field :category
        field :score
      end
  end
  </code></pre>
  
  <p>Using rails_admin gives me a Student filter automatically, however, it searches based on the student_id.
  How do I customize the Student filter in Grades admin view to search students based on its student_code attribute? Using searchable like the docs suggested doesn't give any results.</p>