Question

I’m writing a rails app where there is a model that takes certain measures (height, weight, neck_circumference, etc.)

I want it to work both for metric and imperial measurement system.

My model has a measure_unit attribute that can be both metric or imperial. All the measures will be stored in the database in metric system.

I use the simple_form gem, and I have no clue on how to implement the views so that if a user uses the imperial system his data is converted on the fly.

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

How do I fill the blanks?

EDIT — Found a working solution

I ended up using virtual attributes such as “height_in_cm” and “height_in_in” leaving the entries in the database to be stored in mm.

Here is the code for the 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 the code for the 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 %>  

Answer 1

One solution would be to pass a hidden field back to the controller indicating that the value passed is imperial and needs to be converted into metric for database storage.

So rather than having a different input for metric and imperial measurements you can simply use a single input which defaults to metric and you have to detect for the non-default (imperial):

<%= f.input :height %></br>                                
<%= f.hidden_field :imperial, :value => (@customer.measure_unit.eql? "imperial" ? true : false) %>

Now in your controller action before you have the measurement attribute saved:

if params[:imperial] == true
  #convert imperial measurement to metric (replace @foo with your object)
  @foo.height = convert_to_metric(params[:height]) 
end