How can i override an attribute from another virtual attribute in a model?

Question

I have a virtual attribute start_date and a database column start (datetime).

How can i override the date (but not the time) of start with the date from start_date ?

I tried this in the setter method and the logger says that self.start/start is set, but in the database and also the update log start has still the old date.

# model: db table with columns: id(int), start(datetime)
class DateTimeTest < ActiveRecord::Base

  attr_accessible :start_date, :start

  def start_date=(value)
    logger.debug '>>>>' + start.inspect
    logger.debug '>>>>' + self.start.inspect
    start = DateTime.new(value.to_date.year, value.to_date.month, value.to_date.mday, 15,15,15)
    self.start = DateTime.new(value.to_date.year, value.to_date.month, value.to_date.mday, 15,15,15)
    logger.debug '>>>>' + start.inspect
    logger.debug '>>>>' + self.start.inspect
  end

end

# view
<%= simple_form_for(@datetimetest) do |f| %>
  <%= f.input :start_date, :as => :string %>
  <%= f.input :start %>
  <%= f.button :submit %>
<% end %>

# controller is a base scaffolding controller

In the logs i can see that start is populated:

Parameters: {..., "datetimetest"=>{"start_date"=>"03.05.2012", "start(1i)"=>"2012", "start(2i)"=>"5", "start(3i)"=>"1", "start(4i)"=>"13", "start(5i)"=>"05"}

Answer 1

The DateTime decomposes into year, month, date of month, hour, minute, and second, but does not conveniently break into date and time.

def start_date(value)
  start = DateTime.new(value.year, value.month, value.mday,
                       start.hour, start.minute, start.second)
  logger.debug self.start.inspect
end