rails_admin change boolean default icon true and false instead string 'true' and 'false'

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I did created on one model with boolean field

rails g model saas reserved_seat:boolean

when i add new record it will show only ✓ , ✘ and ‒

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Instead i need to value display neither 'true' or 'false' nor 'ON' or 'OFF' in string

What I did was:

field :reserved_seat do
   pretty_value do
    if bindings[:object].reserved_seat==true
        %{<span style="color:blank;" >ON</span>}.html_safe
      else
        %{<span style="color:blank;" >OFF</span>}.html_safe
    end
  end
 end