EasyRailsMoney

“Young people, nowadays, imagine that money is everything.

Yes, murmured Lord Henry, settling his button-hole in his coat; and when they grow older they know it.”
― Oscar Wilde, The Picture of Dorian Gray and Other Writings

This library provides integration of money gem with Rails.

It provides migration helpers to define a schema with either a single
currency column or a currency column per-money object.

It also provides a ActiveRecord DSL to define that an attribute is a Money object and that it has a default currency

Please open a new issue in the github project issues tracker. You are also more than welcome to contribute to the project :-)

Credits

Have stolen lots of code from money-rails
But database schema, API and tests are written from scratch

money-rails is much more popular and full-featured. Definately try it out. I have actually submitted a PR to that project and it is actively maintained.

I have tried to create a simpler version of money-rails With a better API and database schema, in my opinion.
I created this project to scratch my itch.

Installation

Add this line to your application's Gemfile:

gem 'easy_rails_money'

And then execute:

$ bundle

Or install it yourself as:

$ gem install easy_rails_money

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

Why use the Money gem

In Ruby, Integer and Floats are native types
BigDecimal is like a wrapper over a string

Float is imprecise. storing Money as float will give us round-off
errors. i think will have problems comparing two floats as well

BigDecimal is precise and can handle arbitrary precision
but it is slower

Check https://gist.github.com/deepak/1275050 for a benchmark

So, to represent money we have:

• store as String
• store as decimal in database which Rails typecasts to BigDecimal
• store as integer. Which is what the Money gem does

Money gem wraps:

• currency of a amount
• list of currency conversion rates
• actually convert one currency to another

flipkart.com has a version of the flipkart-money Money gem which uses BigDecimal
check the Money readme as well

Rationale

Let us say you want to store a Rupee Money object in the database

principal = Money.new(100, "inr")

To serialize the values in the database Option 1:

class CreateLoan < ActiveRecord::Migration
  def change
    create_table :loans do |t|
      t.integer       :principal_money
      t.string        :principal_currency
    end
  end
end

Option 2: Another option would be

class CreateLoan < ActiveRecord::Migration
  def change
    create_table :loans do |t|
      t.integer       :principal_as_paise
    end
  end
end

Note that we are storing the base unit in the database. If the amount is in dollars we store in cents or if the amount is in Indian Rupees we store in paise and so on. This is done because FLoats do not have a accurate representation but Integers do. Can store BigDecimal as well but it is slower. This is why the Money gem stores amounts as integer

Watch Rubyconf 2011 Float-is-legacy for more details and read What Every Computer Scientist Should Know About Floating-Point Arithmetic, by David Goldberg, published in March, 1991

We have encoded the currency in the column name. I like it because there is no need to define another column and it is simple. But the disadvantage is that it is inflexible ie. cannot store two currencies and changing the column name in MySQL might require downtime for a big table

So let us go with the first option. The disadvantage is that currency is stored as a string. Integer might be better for storing in the database

Now let us say we want to store multiple columns:

principal = Money.new(100, "inr")
repaid    = Money.new(20, "inr")
npa       = Money.new(10, "inr")

Now we would represent it as

class CreateLoan < ActiveRecord::Migration
  def change
    create_table :loans do |t|
      t.integer       :principal_money
      t.string        :principal_currency
      t.integer       :repaid_money
      t.string        :repaid_currency
      t.integer       :npa_money
      t.string        :npa_currency
    end
  end
end

We are repeating ourself and mostly all currencies for a record will be the same. So we can configure the currency on a per-record basis and write

Option 3:

class CreateLoan < ActiveRecord::Migration
  def change
    create_table :loans do |t|
      t.string        :currency
      t.integer       :principal_money
      t.integer       :repaid_money
      t.integer       :npa_money
    end
  end
end

It might be possible that we set a currency once for the whole app and never change it. But this seems like a nice tradeoff api-wise

Also the column names are suffixed with _money and _currency
We need this for now, to reflect on the database scheme. I Ideally should be able to read the metadata from rails scheme cache.

Usage

ActiveRecord

Only ActiveRecord is supported for now. And has been tested on ActiveRecord 3.x

Migration helpers

If you want to create a table which has some money columns, then you can use the money migration helper

class CreateLoanWithCurrency < ActiveRecord::Migration
  def change
    create_table :loans, force: true do |t|
      t.string :name
      t.money  :principal
      t.money  :repaid
      t.money  :npa
      t.currency
    end
  end
end

If you want to add a money column to an existing table then you can again use the money migration helper

class AddPrincipalToLoan < ActiveRecord::Migration
  def change
    change_table :loans do |t|
      t.money :principal
    end
  end
end

Another option is to use add_money migration helper It is a different DSL style, similar to create_table

class AddPrincipalToLoan < ActiveRecord::Migration
  def up
    add_money :loans, :principal, :repaid, :npa
  end

  def down
    remove_money :loans, :principal, :repaid, :npa
  end
end

add_money helper is revertable, so you may use it inside change migrations. If you writing separate up and down methods, you may use the remove_money migration helper.

The above statements for money and add_money will create two columns. An integer column to store the lower denomination as an integer and a string column to store the currency name.

eg. if we say add_money :loans, :principal Then the following two columns will be created:

1. integer column called principal_money
2. string column called principal_currency

If we want to store $ 100 in this column then:

1. column principal_money will contain the unit in the lower denomination ie. cents in this case. So for $100 it will store 100 * 100 => 100_000 cents
2. column principal_currency will store the currency name ie. usd

Both the amount and currency is needed to create a Money object

Now if we have multiple money columns, then you can choose to have a single currency column

class CreateLoanWithCurrency < ActiveRecord::Migration
  def change
    create_table :loans, force: true do |t|
      t.string :name
      t.money  :principal
      t.money  :repaid
      t.money  :npa
      t.currency
    end
  end
end

This will create a single column for currency:

1. It creates three columns for each of the money columns principal_money, repaid_money and npa_money
2. note that it does not create a currency column for each of the money columns. But a common currency column is created. It is boringly enough called currency

Note that columns are prefixed with _money and _currency And the common currency column is called currency.

It is used to reflect on the database schema ie. to find out the money and currency columns defined.
Right now, none of these choices are customizable.

Defining the Model

If every money column has its own currency column, then we cn define
the model as:

class Loan < ActiveRecord::Base
  attr_accessible :name
  money :principal
  money :repaid
  money :npa
end

The corresponding migration (given above) is:

class CreateLoanWithCurrency < ActiveRecord::Migration
  def change
    create_table :loans, force: true do |t|
      t.string :name
      t.money  :principal
      t.money  :repaid
      t.money  :npa
    end
  end
end

Now if you want a single currency column then:

class Loan < ActiveRecord::Base
  attr_accessible :name

  with_currency(:inr) do
    money :principal
    money :repaid
    money :npa
  end
end

The corresponding migration (given above) is:

class CreateLoanWithCurrency < ActiveRecord::Migration
  def change
    create_table :loans, force: true do |t|
      t.string :name
      t.money  :principal
      t.money  :repaid
      t.money  :npa
      t.currency
    end
  end
end

For such a record, where the single currency is defined. calling currency on a new record will give us the currency. And can define a common currency per-record while creating it

eg:

class Loan < ActiveRecord::Base
  attr_accessible :name

  with_currency(:inr) do
    money :principal
    money :repaid
    money :npa
  end
end

loan = Loan.new
loan.currency # equals Money::Currency.new(:inr)

loan_usd = Loan.new(currency: :usd)
loan_usd.currency # equals Money::Currency.new(:usd)

TODO's

1. Proof-read docs
2. currency is stored as a string. Integer might be better for storing in the database
3. store a snapshot of the exchange rate as well when the record was inserted or if we want to "freeze" the exchange rate per-record
4. specs for migration test the same thing in multiple ways. have a spec helper
5. add Gemfil to test on ActiveRecord 4.x ie. with Rails4 . Add to travis.yml as well
6. configure the _money and _currency prefix and the name of the common currency column
7. check specs tagged as "fixme"
8. cryptographically sign gem
9. test if Memoization in ```MoneyDsl#money`` will make any difference and add a performance test to catch regressions
10. will it make sense to define the money dsl on ActiveModel ?
11. the column names are suffixed with _money and _currency
    We need this for now, to reflect on the database scheme. Ideally should be able to read the metadata from rails scheme cache.
12. see spec tagged with migration.
    if we define a ActiveRecord object with a money column
    "before" the table is defined. Then it will throw
    an error and we will assume that a single
    currency is defined. So always restart the app after the
    migrations are run.
    Any better way ?
    Also the error handling EasyRailsMoney::ActiveRecord::MoneyDsl.single_currency?
    is dependent on the database adapter
    being used, which sucks. can test on other database adapters
    or handle a generic error
13. add typecast for currency column
    right now, it is always a string
    do we want a Money::Currency object back?
    not decided
    see currency_persistence_spec.rb
14. make sure re-opening and redefining money dsl methods work eg. moving from individual currencies to single currency
15. document. methods defined inside activerecord's scope move to a helper, no that its namespace is not polluted
16. two specs tagged with fixme in validates_money_spec failing
17. a version of inclusion_in validator that can compare Symbol and string
18. code parser to lint that multiple money statements are used
    without a with_currency statement. does cane (rubygem) have plugins?