module ActiveRecord::FinderMethods

def fifth

Person.where(["user_name = :u", { u: user_name }]).fifth
Person.offset(3).fifth # returns the fifth object from OFFSET 3 (which is OFFSET 7)
Person.fifth # returns the fifth object fetched by SELECT * FROM people

If no order is defined it will order by primary key.
Find the fifth record. 
def fifth
  find_nth 4
end