Setting & getting virtual attributes in Rails model

Question

I'm looking for a rails-y way to approach the following:

Two datetime attributes in an Event model:

start_at: datetime
end_at:   datetime

I would like to use 3 fields for accessing them in a form:

event_date
start_time
end_time

The problem I'm having is how to keep the actual and the virtual attributes in "sync" so the model can be updated via the form and/or directly via start_at & end_at.

class Event < ActiveRecord::Base
  attr_accessible :end_at, :start_at, :start_time, :end_time, :event_date
    attr_accessor :start_time, :end_time, :event_date

  after_initialize  :get_datetimes # convert db format into accessors
  before_validation :set_datetimes # convert accessors into db format

  def get_datetimes
    if start_at && end_at
      self.event_date ||= start_at.to_date.to_s(:db)   # yyyy-mm-dd 
      self.start_time ||= "#{'%02d' % start_at.hour}:#{'%02d' % start_at.min}" 
      self.end_time   ||= "#{'%02d' % end_at.hour}:#{'%02d' % end_at.min}" 
    end
  end

  def set_datetimes
    self.start_at = "#{event_date} #{start_time}:00"
    self.end_at   = "#{event_date} #{end_time}:00"
  end
end

Which works:

1.9.3p194 :004 > e = Event.create(event_date: "2012-08-29", start_time: "18:00", end_time: "21:00")

 => #<Event id: 3, start_at: "2012-08-30 01:00:00", end_at: "2012-08-30 04:00:00",  created_at: "2012-08-22 19:51:53", updated_at: "2012-08-22 19:51:53"> 

Until setting actual attributes directly (end_at set back to end_time on validation):

1.9.3p194 :006 > e.end_at = "2012-08-30 06:00:00 UTC +00:00"
 => "2012-08-30 06:00:00 UTC +00:00" 
1.9.3p194 :007 > e
 => #<Event id: 3, start_at: "2012-08-30 01:00:00", end_at: "2012-08-30 06:00:00", created_at: "2012-08-22 19:51:53", updated_at: "2012-08-22 19:51:53"> 
1.9.3p194 :008 > e.save
   (0.1ms)  BEGIN
   (0.4ms)  UPDATE "events" SET "end_at" = '2012-08-30 04:00:00.000000', "start_at" = '2012-08-30 01:00:00.000000', "updated_at" = '2012-08-22 20:02:15.554913' WHERE "events"."id" = 3
   (2.5ms)  COMMIT
 => true 
1.9.3p194 :009 > e
 => #<Event id: 3, start_at: "2012-08-30 01:00:00", end_at: "2012-08-30 04:00:00", created_at: "2012-08-22 19:51:53", updated_at: "2012-08-22 20:02:15"> 
1.9.3p194 :010 > 

My assumption is that I also need to customize the "actual" attribute's setters but I'm not sure how to do that w/out screwing up default behavior. Thoughts? Perhaps there a more "Rails-y" "callback-y" way to handle this?

Answer 1

Here's my take. I haven't tested it with ActiveRecord, but I left comments. Hope this helps.

class Event < ActiveRecord::Base

  attr_accessible :end_at, :start_at, :start_time, :end_time, :event_date
  attr_accessor :start_time, :end_time, :event_date

  def start_time
    @start_time || time_attr_from_datetime(start_at)
  end

  def start_time=(start_time_value)
    @start_time = start_time_value
    set_start_at
  end

  def end_time
    @end_time || time_attr_from_datetime(end_at)
  end

  def end_time=(end_time_value)
    @end_time = @end_time_value
    set_end_at
  end

  def event_date
    @event_date || start_at.to_date.to_s(:db)
  end

  def event_date=(event_date_value)
    @event_date = event_date_value
    set_start_at
    set_end_at
  end

  def start_at=(start_at_value)
    write_attribute(:start_at, start_at_value)  # Maybe you need to do write_attribute(:start_at, DateTime.parse(start_at_value)) here ???
    @start_time = time_attr_from_datetime(start_at)
  end

  def end_at=(end_at_value)
    write_attribute(:end_at, end_at_value)  # Maybe you need to do write_attribute(:end_at, DateTime.parse(end_at_value)) here ???
    @end_time = time_attr_from_datetime(end_at)
  end

  private
  def set_start_at
    self.start_at = DateTime.parse("#{event_date} #{start_time}:00")
  end

  def set_end_at
    self.end_at = DateTime.parse("#{event_date} #{end_time}:00")
  end

  def time_attr_from_datetime(datetime)
    "#{'%02d' % datetime.hour}:#{'%02d' % datetime.min}"
  end
end

EDIT: There's a definite pattern to getting and setting start_time and end_time. It could be abstracted a bit with meta-programming, but I thought that would make the example unclear.