form for

Here is what I have done in that situation.

in the form you will want:
  <%= f.text_field :country %>

Then inside your tenant model:
  def country=(country)
    self.country = Country.find_by_name(country)
  end

That way whenever you create or update records, when it comes time to
assign them to an instance of the model you can intercept them and
replace them with the value you really want (which happens to be
elsewhere).