Question

I’m writing an app that acts as a tailor measure form.

The customers model has a lot of attributes that are stored in the database as integers in millimeters.
Since this app will be used both in Europe and in the US I’ll use virtual attributes for showing the user a inches and a centimeter version of the data.

For example for the customer height I have this in my model:

  def height_in_cm
    height / 10
  end

  def height_in_cm=(height)
    self.height = height.to_f  * 10
  end

  def height_in_in
    height * 0.039370
  end

  def height_in_in=(height)
    self.height = height.to_f / 0.039370
  end

And this in my _form view:

  <% if @customer.measure_unit.eql? "imperial" %>
    <%= f.input :height_in_in %></br>
  <% else %>      
    <% if @customer.measure_unit.eql? "metric" %>
      <%= f.input :height_in_cm %></br>
    <% end %>     
  <% end %>

Since as I said I have many attributes my customer model file is becoming extremely long and very error prone.

Is there a dry way to shorten it up?

Answer 1

Encapsulate that logic something like this:

def display_height
  case measure_unit
  when 'imperial' then height_in_in
  when 'metric'   then height_in_cm
  else height_in_in
  end
end

Then your view can just be like this:

<%= @customer.display_height %>

If you are using these same conversion methods in lots of models, extract it out to a module like so:

module HeightConversions

  def height_in_cm
    height / 10
  end

  def height_in_in
    height * 0.039370
  end

  def display_height
    case measure_unit
    when 'imperial' then height_in_in
    when 'metric'   then height_in_cm
    else height_in_in
    end
  end

end

And include it like so in the models where needed:

class Customer
  include HeightConversions
end

EDIT:

Ok, perhaps you need something more like this:

  %w(neck waist arm).each do |name|
    self.class_eval do

      define_method :"#{name}_in_cm" do
        self.send(name) / 10
      end

      define_method :"#{name}_in_cm=" do |n|
        self.send("#{name}=", (n.to_f * 10))
      end

      define_method :"#{name}_in_in" do
        self.send(name) * 0.03
      end

      define_method :"#{name}_in_in=" do |n|
        self.send("#{name}=", (n.to_f / 0.039370))
      end

    end
  end

This is usually called metaprogramming. Here’s a nice concise article on different ways to do it in Rails:
http://www.trottercashion.com/2011/02/08/rubys-define_method-method_missing-and-instance_eval.html