Ruby on Rails: A way to save currency into a column within a database-table

Question

I have a database-table cost with the columns amount and currency. I search for a solution to save a currency into this column.

What do you suggest: Which data type for the column currency and what values should I save there (iso-code, currency symbol ...)? Or maybe there is already a data type Currency in ruby like in java?

This question has been discussed [here](http://stackoverflow.com/questions/1019939/ruby-on-rails-best-method-of-handling-currency-money). Hope that helps. — Thierry M., Jan 09 '16 at 13:49
Indeed they discuss how to handle currency while displaying them within a view using number_to_currency, but my issue is to find a practical way how to save currency into a database table. — software knife, Jan 09 '16 at 14:03

score 4 · Answer 1 · edited May 23 '17 at 11:53

Here are all the Rails 4 (ActiveRecord migration) datatypes:

:binary
:boolean
:date
:datetime
:decimal
:float
:integer
:primary_key
:references
:string
:text
:time
:timestamp

Source: Rails 4: List of available datatypes

As you can see there is no data type currency. I suggest you to create a decimal column for the amount and an integer for the currency.

Why integer? Well because you can set an enum for that column in your model like this:

enum currency: [:dollar]

This creates a method with the same name as the enum, so let's say you have a cost variable:

cost.dollar?

This will return true or false depending on the value.

Also with this you can easily create some case to make all the changes in currency you like:

case cost.currency
when Cost.currencies[:dollar]
  # Do something
end

Always compare this column with Cost.currencies[:symbol_of_currency] instead of the number (enums work just like C, 0 would be the first element), this will allow you to change the order of the elements inside the enum or add new ones without changing the functionality of your app.

Also, this allows you to create the rows in the database like this:

Cost.create(currency: :dollar, amount: 30.0, ...)

to save a currency as an integer and use enums seems to work fine. But I thought to find something like that Currency.new(cost.currency). Assumed there is already a ruby class currency. — software knife, Jan 09 '16 at 14:10
You should never save an amount as a floating point, as you may loose precision during calculations. — Simone Carletti, Jan 09 '16 at 14:15
You are right, eventually you could get weird values.. I edited to decimal — alebian, Jan 09 '16 at 14:20

Simone Carletti · Accepted Answer · 2016-01-09T14:28:41.630

In general, you save the amounts as integers, because floating point are known to have rounding errors that are definitely not suitable for representing money.

Because money can have fractions, it's quite common that the integer represents the amount in cent, so that you can easily perform operations on the amounts, then divide by /100 to get the value as dollars.

The money gem is the most popular library in Ruby to deal with currencies and money. Internally, the library uses exactly the same approach I just described. Actually, the library was moved in the past to use BigDecimal which are a Ruby-specific library for dealing with arbitrary precision integer arithmetic.

If you want to use it in Rails, the money-rails gem is a quick solution. However, you are not required to use this gem, nor the money gem, especially if your need is limited to that specific model and basic operations.

It's your choice. However, you should really represent amounts in integers. As for currencies, you can use simple varchar fields and use the 3-char standard representation.

Ruby on Rails: A way to save currency into a column within a database-table

2 Answers2