Question

In my Rails 3.2 app, I want to populate some fields based on calculations where the field values users enter are the variables. However, with my current code, the calculation seems to only work based on the values already in the database – it doesn’t calculate correctly on the initial save, but it will calculate correctly if I go back in the record and save it a second time.

I have these four fields in my model (Trade):

1. entry_price
2. exit_price
3. percent_result
4. dollar_result

The user creates a trade with an entry price, and then later edits the trade with the exit_price. When the exit_price is entered, the app should calculate percent_result and dollar_result. However, right now, these result fields are not populating correctly on the first update – it seems to be because it doesn’t read the exit_price from the field (when a user enters it in the form), only once it is saved in the DB.

What is going wrong in my controller?

my controller:

def update
  @trade = Trade.find(params[:id])
  exit_price = params[:trade][:exit_price]

  if !exit_price.blank?
    @trade.percent_result = ((exit_price.to_f - @trade.entry_price)/@trade.entry_price) * 100
    @trade.dollar_result = exit_price.to_f - @trade.entry_price 
  end

  params[:trade][:exit_date] = Date.strptime(params[:trade][:exit_date], '%m/%d/%Y') unless params[:trade][:exit_date].blank?
  params[:trade][:entry_date] = Date.strptime(params[:trade][:entry_date], '%m/%d/%Y') unless params[:trade][:entry_date].blank?
  respond_to do |format|
    if @trade.update_attributes(params[:trade])
      format.html { redirect_to @trade, :flash => {:share =>"Your trade was successfully updated.  Don't forget to share it with your friends, so you can profit together!"} }
      format.json { head :no_content }
    else
      format.html { render action: "edit" }
      format.json { render json: @trade.errors, status: :unprocessable_entity }
    end
  end
end

the view

<%= simple_form_for(@trade, :html=>{:class=> "form-horizontal well"})  do |f| %>  
  <%= f.text_field :entry_price, :class=>"input-small" %>
  <%= f.text_field :exit_price, :class=>"input-small" %>

  <%= submit_tag "Edit Trade" %>
<% end %>

Answer 1

This would probably be better accomplished with a before_save filter in your model.

Add

before_save :calculate_results

to the top of your model and then define

def calculate_results
    unless self.exit_price.blank? || self.entry_price.blank?
        self.percent_result = ((self.exit_price - self.entry_price)/self.entry_price) * 100
        self.dollar_result = self.exit_price - self.entry_price 
    end
end

in your model as well. Taking this approach ensures that your results will always be consistent with your values for entry and exit price. Enforcing this in the controller violates the Rails principle of “thick model and thin controller” and may also lead to data consistency issues.

An even more consistent way of doing this would be to define dollar_result and percent_result as methods in your model. As your model is now, you have dollar_result stored in the database even though it is a derived value. As a general rule, you should only have one representation of each piece of data whereas here you have two. A helper method might look something like

def dollar_result
    self.exit_price - self.entry_price unless self.exit_price.blank? || self.entry_price.blank?
end

You would define a similar method for percent_result. Using this method, you can guarantee that all of your data is consistent because it only has one, canonical representation in the system.