money-rails, Money#== supports only zero numerics

Question

I'm using the money-rails gem in my Rails application. It have worked perfectly until now, but I've recently started getting the following error:

Money#== supports only zero numerics

I'm not really sure what caused this or how to solve it. I have recently run bundle update so I guess that something have been updated. My gemfile looks like this:

gem 'money-rails', '~>1'
gem 'eu_central_bank', "~>1.3.0"

My implementation looks like this:

# model
monetize :price_in_cents

# fetch / converting currencies
eu_bank = EuCentralBank.new
Money.default_bank = eu_bank
eu_bank.update_rates
converted_price = eu_bank.exchange_with(Money.new(price_to_convert * 100, from_currency), to_currency)

This have as I mentioned worked before, so I'm not sure what is breaking it.

Any ideas?

Update

To test I tried with the following.

money = Money.new(100, from_currency)

Then I got the same error as before. But if I tried:

money = Money.new(0, from_currency)

It seems to work. I find that a bit strange.

Update

Here is the backtrace from when I try to save the record:

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Answer 1

The error comes from a numericality validation on your model.

Money gem does not allow to compare a money object with a Number, unless thee number is zero. Otherwise it expects you to compare Money with Money.

In irb you can try:

```ruby

2.5.1 :006 > Money.new(1000, "USD") != Money.new(1000, "USD")
 => false 

2.5.1 :007 > Money.new(1000, "USD") != 1000
Traceback (most recent call last):
        4: from /Users/andi/.rvm/rubies/ruby-2.5.1/bin/irb:11:in `<main>'
        3: from (irb):7
        2: from (irb):7:in `!='
        1: from /Users/andi/.rvm/gems/ruby-2.5.1/gems/money-6.12.0/lib/money/money/arithmetic.rb:70:in `=='
ArgumentError (Money#== supports only zero numerics)

2.5.1 :008 > Money.new(1000, "USD") != 0
 => true 

```

I believe that this error makes sense, as you can't really compare an arbitrary number with an amount of money in a given currency.

The money-rails gem again comes with it's own validators:

https://github.com/RubyMoney/money-rails#numericality-validation-options

Your backtrace shows that you are using the rails numericality validator instead.

Answer 2

Money Rails automatically adds "_cents" to the attribute you are monetizing. Then it creates an instance of that model.price which includes

<Money fractional:1500 currency:DKK>

Hence, you cannot do model.price + 10 as it would cause the error you are getting. You may therefore want to monetize price_to_convert, or any other value you are working with so that you can sum them up, etc.

Create a new migration file e.g. rails g migration AddMoneytizeToOrders

change migration file so that it reads as follow (change column/attribute to your values)

class AddMonetizeToOrder < ActiveRecord::Migration[6.0]
  def change
    add_monetize :orders, :price_to_convert, currency: { present: false }
  end
end

In your model you must include

monetize :price_to_convert_cents

then you can work with e.g.(rails c) Order.last.price_to_convert + Model.last.price, etc.

Please note that you cannot use self.[:price_to_convert] as money rails wouldn't understand it and you would get the error

money rails ActiveModel::MissingAttributeError (can't write unknown attribute...

Therefore use self.price_to_convert instead

Answer 3

You might also come across this error when you are writing specs:

expect(order.subtotal).to eq 123.45
# ArgumentError:
#   Money#== supports only zero numerics

expect(order.subtotal).to eq Money.new(12345)
# works!