Extend Capybara's #set and #fill_in methods to support select boxes. Save to Rails directory config/initializers/

capybara_ext.rb

require 'capybara'
module Capybara
  module Node
    module ElementExt
      def set(value, fill_options={})
        if self.tag_name == 'select'
          opt = find(:option, value)
          if opt.blank?
            warn "Could not find option '#{value}' in '#{self.inspect}"
          else
            opt.select_option
          end
        else
          super(value, fill_options)
        end
      end
    end
    module ActionsExt
      def fill_in(name, options={})
        begin
          super(name, with: options[:with])
        rescue Capybara::ElementNotFound
          find_field(name).set(options[:with])
        end
      end
    end

    Element.prepend ElementExt
    Base.prepend ActionsExt
  end
end

Updated using Module#prepend to address deprecation of alias_method_chain