Question

In Ruby on Rails I have a collection of data which needs to be filtered to get the most recently created ones of each month. What is the best optimized way to do this?

e.g

["2012-1-2","2012-1-18", "2012-1-5", "2012-2-15","2012-2-23","2012-2-4"]

result should be

["2012-1-18, "2012-2-23", ..]

Answer 1

def last_in_month(dates)
  dates = dates.map {|date_string| Date.parse(date_string)}
  grouped_by_month = dates.group_by {|date| date.month}
  grouped_by_month.map do |month, dates_in_month|
    dates_in_month.max_by {|d| d.day}
  end
end

last_in_month(your_nested_arrays.flatten)

Returns dates objects.

Transform to strings again 🙂

last_in_month(your_nested_arrays.flatten).map {|d| d.to_s(:db)}