example.rb

class JuryReview

  # some mongo fields here, not called round_number nor rapper_slug
  
  # Now let's have a getter and setter, these are the names for the form fields as well, e.g.
  #   f.select :rapper  - This should render a select with <option value="ID OF THE RAPPER OR SLUG">WHATEVER</option>
  #   f.input :round_number
  attr_accessor :round_number, :rapper
  
  # Implement the setter first
  def round_number=(n)
    self.round = Round.where(number: n).first
  end
  
  # Since you have replaced the embbeded_in (!!!) you can query the rapper directly without the round
  # This is important because you can't guarantee that the round is saved before the rapper is being set
  def rapper=(id_or_slug)
    self.rapper = Rapper.where(slug: id_or_slug).first
  end
  
  ....
  
end