@Lathi.net

Simple Helper Objects

Posted by Doug Thu, 13 Oct 2005 21:36:14 GMT

I thought it was a one off, but I’ve got several data classes in my Rails projects that aren’t derived from ActiveRecord. I’m using these to encapsulate some data and logic that doesn’t necessarily need to be it’s own table associated with some ActiveRecord model. Let me give a concrete example.

I now have three models that need to record a time. It’s not a DateTime or a Date. It’s just a time: “7:00 am”. I need to be able to sort these models based on that 12 hour time. I suppose I could have just stored the time as a 24-hour time integer. Even if I had done that, there still would have been some helper methods needed to convert to a 12 hour clock string. So, this approach seems as good as another.

WARNING: I’m proving to be pretty bad at naming classes. This example illustrates that point.

So I have a TimeString class. It’s got hour, minute, and ampm accessors. It also knows how to dump and load itself to/from YAML. My ActiveRecord models have special accessors to contain one of these TimeStrings. I have a before_save filter to convert the contained TimeString to a YAML string and save that in the database. In essense I have a column in my database that acts_as_object. I think a component to implement this feature would be generally useful.

The code looks like this:

class TimeString
  attr_settor: :hour, :minute, :ampm

  # some class implementation details

  def dump
    YAML.dump(self)
  end

  def TimeString.load(string)
    YAML.load(string)
  end
end

And it’s used like this:

class MyClass < ActiveRecord::Base
  attr_accessor :time

  before_save :save_time
  def save_time
    self.time_obj = self.time.dump
  end

  def time
    @timex ||= TimeString.load(self.time_obj)
  end

  def time=(obj)
    self.time_obj = obj.dump
  end
end

This assumes you have a database column called time_obj. That column contains the YAML dump of the object. The time method is an accessor that returns a real TimeString object. The time= method takes a TimeString object and dumps it to the database column.

The technique is both neat and hackish at the same time. It’s proved to be useful. The pattern is fairly generic. This example uses my TimeString class that you may or may not think is necessary, but could really apply to any class you wanted to contain. The only thing that’s proved to vary is the obj= method on the ActiveRecord model. That varies based on how you want to be able to set the contained object. In some cases I want to pass in an already instantiated object. In others I want to be able to pass in some parameters to create the contained object.

I’m not exactly sure how to go about doing it, but I like the idea of converting this into an acts_as_object class method that would set up the appropriate stuff.