Question

I have a model with a virtual attribute for a time interval:

attr_accessible :description, :time_end, :time_start, :duration
belongs_to :timesheet

def duration
  if attribute_present?("time_start") and attribute_present?("time_end")
    ChronicDuration.output(self.time_end - self.time_start) 
  else
    ChronicDuration.output(0)
  end
end

def duration=(d)
  self.time_end = self.time_start + d
end

However, when creating a new object, Rails tries to set duration before start, leading to an error. How can I make sure that duration is set after start?

error:

undefined method `+' for nil:NilClass

params:

{"utf8"=>"✓",
 "authenticity_token"=>"dg+CysIxZORyV3cwvD+LdWckFdHgecGDFDBNOip+iKo=",
 "entry"=>{"time_start"=>"now",
 "duration"=>"2h",
 "description"=>""},
 "commit"=>"Create Entry"}

Answer 1

A few things

• Worth reading about: and vs && in ruby – http://devblog.avdi.org/2010/08/02/using-and-and-or-in-ruby/

• some alternates to using attribute_present? method

  # opposite of blank? - http://api.rubyonrails.org/classes/Object.html#method-i-present-3F
  if time_start.present? && time_end.present?
  
  # short hand syntax for present?
  if time_start? && time_end?
  

I don’t think your problem is with duration being set before time_start, assuming time_start is a datetime or time database type
try this in rails console

entry = Entry.new
entry.time_start = "now" 
# => "now"
entry.time_start
# => nil

you are passing strings into time objects and rails / ruby just sets the value to nil.
If time_end and time_start were strings I still don’t think your code would give you the result you want?

def duration=(d)
  self.time_end = self.time_start + d
end

# params: time_start = "now"
# params: duration = "2h"
# time_end would be: now2h

if I am wrong about duration= running before time_start is set, an alternative would be something like this using a before_save callback

class Entry < ActiveRecord::Base
  before_save :set_time_end

  attr_accessor :duration
  attr_accessible :description, :time_end, :time_start, :duration

  belongs_to :timesheet

  def duration
    if time_start? && time_end?
      ChronicDuration.output(self.time_end - self.time_start) 
    else
      ChronicDuration.output(0)
    end
  end

  def set_time_end
    return nil if time_start.blank?
    self.time_end = self.time_start + self.duration
  end
end