What is the best method of handling currency/money?

I'm working on a very basic shopping cart system.

I have a table items that has a column price of type integer.

I'm having trouble displaying the price value in my views for prices that include both Euros and cents. Am I missing something obvious as far as handling currency in the Rails framework is concerned?

Solution 1:

You'll probably want to use a DECIMAL type in your database. In your migration, do something like this:

# precision is the total number of digits
# scale is the number of digits to the right of the decimal point
add_column :items, :price, :decimal, :precision => 8, :scale => 2

In Rails, the :decimal type is returned as BigDecimal, which is great for price calculation.

If you insist on using integers, you will have to manually convert to and from BigDecimals everywhere, which will probably just become a pain.

As pointed out by mcl, to print the price, use:

number_to_currency(price, :unit => "€")
#=> €1,234.01

Solution 2:

Here's a fine, simple approach that leverages composed_of (part of ActiveRecord, using the ValueObject pattern) and the Money gem

You'll need

The Money gem (version 4.1.0)
A model, for example Product
An integer column in your model (and database), for example :price

Write this in your product.rb file:

class Product > ActiveRecord::Base

  composed_of :price,
              :class_name => 'Money',
              :mapping => %w(price cents),
              :converter => Proc.new { |value| Money.new(value) }
  # ...

What you'll get:

Without any extra changes, all of your forms will show dollars and cents, but the internal representation is still just cents. The forms will accept values like "$12,034.95" and convert it for you. There's no need to add extra handlers or attributes to your model, or helpers in your view.
product.price = "$12.00" automatically converts to the Money class
product.price.to_s displays a decimal formatted number ("1234.00")
product.price.format displays a properly formatted string for the currency
If you need to send cents (to a payment gateway that wants pennies), product.price.cents.to_s
Currency conversion for free

Solution 3:

Common practice for handling currency is to use decimal type. Here is a simple example from "Agile Web Development with Rails"

add_column :products, :price, :decimal, :precision => 8, :scale => 2

This will allow you to handle prices from -999,999.99 to 999,999.99
You may also want to include a validation in your items like

def validate 
  errors.add(:price, "should be at least 0.01") if price.nil? || price < 0.01 
end

to sanity-check your values.

Solution 4:

Just a little update and a cohesion of all the answers for some aspiring juniors/beginners in RoR development that will surely come here for some explanations.

Working with money

Use :decimal to store money in the DB, as @molf suggested (and what my company uses as a golden standard when working with money).

# precision is the total number of digits
# scale is the number of digits to the right of the decimal point
add_column :items, :price, :decimal, precision: 8, scale: 2

Few points:

:decimal is going to be used as BigDecimal which solves a lot of issues.
precision and scale should be adjusted, depending on what you are representing
- If you work with receiving and sending payments, precision: 8 and scale: 2 gives you 999,999.99 as the highest amount, which is fine in 90% of cases.
- If you need to represent the value of a property or a rare car, you should use a higher precision.
- If you work with coordinates (longitude and latitude), you will surely need a higher scale.

How to generate a migration

To generate the migration with the above content, run in terminal:

bin/rails g migration AddPriceToItems price:decimal{8-2}

bin/rails g migration AddPriceToItems 'price:decimal{5,2}'

as explained in this blog post.

Currency formatting

KISS the extra libraries goodbye and use built-in helpers. Use number_to_currency as @molf and @facundofarias suggested.

To play with number_to_currency helper in Rails console, send a call to the ActiveSupport's NumberHelper class in order to access the helper.

For example:

ActiveSupport::NumberHelper.number_to_currency(2_500_000.61, unit: '€', precision: 2, separator: ',', delimiter: '', format: "%n%u")

gives the following output

2500000,61€

Check the other options of number_to_currency helper.

Where to put it

You can put it in an application helper and use it inside views for any amount.

module ApplicationHelper    
  def format_currency(amount)
    number_to_currency(amount, unit: '€', precision: 2, separator: ',', delimiter: '', format: "%n%u")
  end
end

Or you can put it in the Item model as an instance method, and call it where you need to format the price (in views or helpers).

class Item < ActiveRecord::Base
  def format_price
    number_to_currency(price, unit: '€', precision: 2, separator: ',', delimiter: '', format: "%n%u")
  end
end

And, an example how I use the number_to_currency inside a contrroler (notice the negative_format option, used to represent refunds)

def refund_information
  amount_formatted = 
    ActionController::Base.helpers.number_to_currency(@refund.amount, negative_format: '(%u%n)')
  {
    # ...
    amount_formatted: amount_formatted,
    # ...
  }
end