Issue Performing Mathematical Operations on Money Objects - "Unknown Method "exchange_to" for 0:fixnum" [Rails 4] [Money Gem]

Question

I currently have the money gem installed via money-rails and it's working great. I have a monetized object called :default_price and :default_price_cents and am using a :currency column to define the currencies. I also have a :default_packs column which is a user defined integer that's monetized, and a Pack model which stores a new entry every day with a :date column and a :amount column.

Here's my user.rb

  has_many :packs

  register_currency :usd

  monetize :default_price_cents, with_model_currency: :currency
  monetize :default_price, with_model_currency: :currency
  monetize :daily_saving_potential, with_model_currency: :currency
  monetize :total_saving_potential, with_model_currency: :currency
  monetize :total_money_spent, with_model_currency: :currency
  monetize :total_savings, with_model_currency: :currency

  def daily_saving_potential
    return default_price * default_packs
  end

  def total_saving_potential
    days = self.packs.count
    return days * daily_saving_potential
  end

  def total_money_spent
    amount = self.packs.map(&:amount).sum
    return amount
  end

  def total_savings
    return total_saving_potential - total_money_spent
  end

The Problem

total_money_spent sums all the entries of the :amount field from my Pack model. The problem arises that when total_savings is called upon in my view, I get an error saying Unknown Method "exchange_to" for 0:fixnum.

The interesting thing is that if I use :default_price_cents instead of :default_price in daily_saving_potential, then I don't get the error, but my custom defined fields in the model display the default currency instead of the currency defined by the user, whereas the default_price field displays the right currency defined by the user. Perhaps the issue has to do with the fact that I'm multiplying money objects with a non-money object from my Pack model? Regardless, the issue arises when I subtract the total_money_spent from total_saving_potential

Let me know if you need more information, and all help is greatly appreciated!

Answer 1

I haven't worked with this gem, but it doesn't look like you've followed the read-me for the money-rails gem

https://github.com/RubyMoney/money-rails

Assuming you have a field in the model :default_price_cents you can do...

  monetize :default_price_cents, with_model_currency: :currency

... and this will automatically give you a monetzed field called ':default_price`. You don't need to define it and you don't need to "monetize" it, it's already a money attribute.

Similarly, you should be creating methods called daily_saving_potential_cents and total_saving_potential_cents and total_money_spent_cents etc etc etc. You will automatically have a method daily_saving_potential, and total_saving_potential and so on.

In your method calculations use the raw (.._cents) attributes.