user.profile_name #=> should return "user#{user.id}" if nil

# model User.rb
def get_identifier
   profile_name ? profile_name : "user#{id}"
end

id, user = params[:identifier], nil
if id[0..3] == "user"
   user = User.find_by_id(id[3..-1].to_num)
else
   user = User.find_by profile_name: id

class User
  def self.find_by_profile_name(profile_name)
    where(profile_name: profile_name).first || find_by_default_profile_name(profile_name)
  end

  def self.find_by_default_profile_name(profile_name)
    where(id: DefaultProfileName.from_profile_name(profile_name).id)
  end
end


class DefaultProfileName
  attr_accessor :id

  def self.from_profile_name(profile_name)
    new(profile_name.sub('user', ''))
  end

  def initialize(id)
    self.id = id
  end

  def to_s
    "user#{id}"
  end
end