module ActiveRecord::FinderMethods

def last(limit = nil)

[#, #, #]

and not:

[#, #, #]

Take note that in that last case, the results are sorted in ascending order:

Person.last(3) # returns the last three objects fetched by SELECT * FROM people.
Person.order("created_on DESC").offset(5).last
Person.where(["user_name = ?", user_name]).last
Person.last # returns the last object fetched by SELECT * FROM people

If no order is defined it will order by primary key.
Find the last record (or last N records if a parameter is supplied). 
def last(limit = nil)
  if limit
    if order_values.empty? && primary_key
      order(arel_table[primary_key].desc).limit(limit).reverse
    else
      to_a.last(limit)
    end
  else
    find_last
  end
end