# Introduction to Money
Money implements a set of functions to store, retrieve and perform arithmetic
on a `%Money{}` type that is composed of a currency code and a currency amount.
Money is opinionated in the interests of serving as a dependable library
that can underpin accounting and financial applications. In its initial
release it can be expected that this contract may not be fully met.
How is this opinion expressed?
1. Money must always have both a amount and a currency code.
2. The currency code must always be a valid ISO4217 code.
3. Money arithmetic can only be performed when both operands are of the same currency.
4. Money amounts are represented as a `Decimal`.
5. Money can be serialised to the database as a composite Postgres type that includes both the amount and the currency. Therefore for Ecto serialization Postgres is assumed as the data store. Serialization is entirely optional.
6. All arithmetic functions work on a `Decimal`. No rounding occurs automatically (unless expressly called out for a function, as is the case for `Money.split/2`).
7. Explicit rounding obeys the rounding rules for a given currency. The rounding rules are defined by the Unicode consortium in its CLDR repository as implemented by the hex package `ex_cldr`. These rules define the number of fractional digits for a currency and the rounding increment where appropriate.
8. Money output string formatting output using the hex package [ex_cldr](https://hex.pm/packages/ex_cldr) that correctly rounds to the appropriate number of fractional digits and to the correct rounding increment for currencies that have minimum cash increments (like the Swiss Franc and Australian Dollar)
## Exchange rates and currency conversion
Money includes a process to retrieve exchange rates on a periodic basis. These exchange rates can then be used to support currency conversion. This service is started by default and will attempt to retrieve exchange rates every 5 minutes.
By default, exchange rates are retrieved from [Open Exchange Rates](http://openexchangerates.org) however any module that conforms to the `Money.ExchangeRates` behaviour can be configured.
An optional callback module can be defined. This module defines a `rates_retrieved/2` function that is invoked upon every successful retrieval of exchange rates.
## Configuration
`Money` provides a set of configuration keys to customize behaviour. The default configuration is:
config :ex_money,
exchange_rate_service: false,
exchange_rates_retrieve_every: 360_000,
api_module: Money.ExchangeRates.OpenExchangeRates,
open_exchange_rates_app_id: nil,
callback_module: Money.ExchangeRates.Callback
log_failure: :warn,
log_info: :info,
log_success: nil
These keys are are defined as follows:
* `exchange_rate_service` is a boolean that determines whether to start the exchange rate retrieval service. The default it `false`.
* `exchange_rates_retrieve_every` defines how often the exchange rates are retrieved in milliseconds. The default is 5 minutes (300,000 milliseconds)
* `api_module` identifies the module that does the retrieval of exchange rates. This is any module that implements the `Money.ExchangeRates` behaviour. The default is `Money.ExchangeRates.OpenExchangeRates`
* `open_exchange_rates_app_id` defines the `app_id` that is required to use the Open Exchange Rates api
* `callback_module` defines a module that follows the `Money.ExchangeRates.Callback` behaviour whereby the function `rates_retrieved/2` is invoked after every successful retrieval of exchange rates. The default is `Money.ExchangeRates.Callback`.
* `log_failure` defines the log level at which api retrieval errors are logged. The default is `:warn`
* `log_success` defines the log level at which successful api retrieval notifications are logged. The default is `nil` which means no logging.
* `log_info` defines the log level at which service startup messages are logged. The default is `info`.
Keys can also be configured to retrieve values from environment variables. This lookup is done at runtime to facilitate deployment strategies. If the value of a configuration key is `{:system, "some_string"}` then `"some_string"` is interpreted as an environment variable name which is passed to `System.get_env/2`. An example configuration might be:
config :ex_money,
exchange_rate_service: {:system, "RATE_SERVICE"},
exchange_rates_retrieve_every: {:system, "RETRIEVE_EVERY"},
open_exchange_rates_app_id: {:system, "OPEN_EXCHANGE_RATES_APP_ID"}
## Why yet another Money package?
* Fully localized formatting and rounding using [ex_cldr](https://hex.pm/packages/ex_cldr)
* Provides serialization to Postgres using a composite type that keeps both the currency code and the amount together removing a source of potential error
* Uses the `Decimal` type in Elixir and the Postgres `numeric` type to preserve precision
## Examples
### Creating a %Money{} struct
iex> Money.new(:USD, 100)
#Money<:USD, 100>
iex> Money.new("CHF", 130.02)
#Money<:CHF, 130.02>
iex> Money.new("thb", 11)
#Money<:THB, 11>
The canonical representation of a currency code is an `atom` that is a valid
[ISO4217](http://www.currency-iso.org/en/home/tables/table-a1.html) currency code. The amount of a `%Money{}` is represented by a `Decimal`.
### Optional ~M sigil
An optional sigil module is available to aid in creating %Money{} structs. It needs to be imported before use:
import Money.Sigil
~M[100]USD
#> #Money<:USD, 100>
### Localised String formatting
See also `Money.to_string/2` and `Cldr.Number.to_string/2`):
iex> Money.to_string Money.new("thb", 11)
"THB11.00"
iex> Money.to_string Money.new("USD", 234.467)
"$234.47"
iex> Money.to_string Money.new("USD", 234.467), format: :long
"234.47 US dollars"
Note that the output is influenced by the locale in effect. By default the localed used is that returned by `Cldr.get_local/0`. Its default value is "en". Additional locales can be configured, see `Cldr`. The formatting options are defined in `Cldr.Number.to_string/2`.
### Arithmetic Functions
See also the module `Money.Arithmetic`:
iex> m1 = Money.new(:USD, 100)
#Money<:USD, 100>
iex> m2 = Money.new(:USD, 200)
#Money<:USD, 200>
iex> Money.add(m1, m2)
#Money<:USD, 300>
iex> m3 = Money.new(:AUD, 300)
#Money<:AUD, 300>
iex(11)> Money.add(m1, m3)
** (ArgumentError) Cannot add two %Money{} with different currencies. Received :USD and :AUD.
(ex_money) lib/money.ex:46: Money.add/2
# Split a %Money{} returning the a dividend and a remainder. All
# operations respect the number of fractional digits defined for a currency
iex> m1 = Money.new(:USD, 100)
#Money<:USD, 100>
iex> Money.split(m1, 3)
{#Money<:USD, 33.33>, #Money<:USD, 0.01>}
# Rounding applies the currency definitions of CLDR as implemented in
# the hex package [ex_cldr](https://hex.pm/packages/ex_cldr)
iex> Money.round Money.new(:USD, 100.678)
#Money<:USD, 100.68>
iex> Money.round Money.new(:JPY, 100.678)
#Money<:JPY, 101>
### Currency Conversion
A `%Money{}` struct can be converted to another currency using `Money.to_currency/3`. For example:
iex> Money.to_currency Money.new(:USD,100), :AUD
#Money<:AUD, 136.4300>
iex> Money.to_currency Money.new(:USD,100), :ZIP
** (Money.UnknownCurrencyError) The currency code :ZIP is not known
iex> Money.to_currency Money.new(:USD,100), :XXX
** (Money.ExchangeRateError) No exchange rate is available for currency :XXX
A user-defined map of exchange rates can also be supplied:
iex> Money.to_currency Money.new(:USD,100), :AUD, %{USD: Decimal.new(1.0), AUD: Decimal.new(1.3)}
#Money<:AUD, 130>
### Financial Functions
A set of financial functions are available in the module `Money.Financial`. These are `use`d in the `Money` module. See `Money` for the available functions.
## Serializing to a Postgres database with Ecto
First generate the migration to create the custom type:
mix money.gen.migration
* creating priv/repo/migrations
* creating priv/repo/migrations/20161007234652_add_money_with_currency_type_to_postgres.exs
Then migrate the database:
mix ecto.migrate
07:09:28.637 [info] == Running MoneyTest.Repo.Migrations.AddMoneyWithCurrencyTypeToPostgres.up/0 forward
07:09:28.640 [info] execute "CREATE TYPE public.money_with_currency AS (currency_code char(3), amount numeric(20,8))"
07:09:28.647 [info] == Migrated in 0.0s
Create your database migration with the new type (don't forget to `mix ecto.migrate` as well):
```elixir
defmodule MoneyTest.Repo.Migrations.CreateLedger do
use Ecto.Migration
def change do
create table(:ledgers) do
add :amount, :money_with_currency
timestamps()
end
end
end
```
Create your schema using the `Money.Ecto.Composite.Type` ecto type:
```elixir
defmodule Ledger do
use Ecto.Schema
schema "ledgers" do
field :amount, Money.Ecto.Composite.Type
timestamps()
end
end
```
Insert into the database:
iex> Repo.insert %Ledger{amount: Money.new(:USD, 100)}
[debug] QUERY OK db=4.5ms
INSERT INTO "ledgers" ("amount","inserted_at","updated_at") VALUES ($1,$2,$3)
[{"USD", #Decimal<100>}, {{2016, 10, 7}, {23, 12, 13, 0}}, {{2016, 10, 7}, {23, 12, 13, 0}}]
Retrieve from the database:
iex> Repo.all Ledger
[debug] QUERY OK source="ledgers" db=5.3ms decode=0.1ms queue=0.1ms
SELECT l0."amount", l0."inserted_at", l0."updated_at" FROM "ledgers" AS l0 []
[%Ledger{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">, amount: #<:USD, 100.00000000>,
inserted_at: ~N[2017-02-21 00:15:40.979576],
updated_at: ~N[2017-02-21 00:15:40.991391]}]
## Serializing to a MySQL (or other non-Postgres) database with Ecto
Since MySQL does not support composite types, the `:map` type is used which in MySQL is implemented as a `JSON` column. The currency code and amount are serialised into this column.
### Notes:
1. In order to preserve precision of the decimal amount, the amount part of the `%Money{}` struct is serialised as a string. This is done because JSON serializes numeric values as either `integer` or `float`, neither of which would not preserve precision of a decimal value.
2. The precision of the serialized string value of amount is affected by the setting of `Decimal.get_context`. The default is 28 digits which should cater for your requirements.
3. Serializing the amount as a string means that SQL query arithmetic and equality operators will not work as expected. You may find that `CAST`ing the string value will restore some of that functionality. For example: `CAST(amount_map AS DECIMAL(20, 8)) AS amount`.
```elixir
defmodule MoneyTest.Repo.Migrations.CreateLedger do
use Ecto.Migration
def change do
create table(:ledgers) do
add :amount, :map
timestamps()
end
end
end
```
Create your schema using the `Money.Ecto.Map.Type` ecto type:
```elixir
defmodule Ledger do
use Ecto.Schema
schema "ledgers" do
field :amount, Money.Ecto.Map.Type
timestamps()
end
end
```
Insert into the database:
iex> Repo.insert %Ledger{amount_map: Money.new(:USD, 100)}
[debug] QUERY OK db=25.8ms
INSERT INTO "ledgers" ("amount_map","inserted_at","updated_at") VALUES ($1,$2,$3)
RETURNING "id" [%{amount: "100", currency: "USD"},
{{2017, 2, 21}, {0, 15, 40, 979576}}, {{2017, 2, 21}, {0, 15, 40, 991391}}]
{:ok,
%MoneyTest.Thing{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">,
amount: nil, amount_map: #Money<:USD, 100>, id: 3,
inserted_at: ~N[2017-02-21 00:15:40.979576],
updated_at: ~N[2017-02-21 00:15:40.991391]}}
Retrieve from the database:
iex> Repo.all Ledger
[debug] QUERY OK source="ledgers" db=16.1ms decode=0.1ms
SELECT t0."id", t0."amount_map", t0."inserted_at", t0."updated_at" FROM "ledgers" AS t0 []
[%Ledger{__meta__: #Ecto.Schema.Metadata<:loaded, "ledgers">,
amount_map: #Money<:USD, 100>, id: 3,
inserted_at: ~N[2017-02-21 00:15:40.979576],
updated_at: ~N[2017-02-21 00:15:40.991391]}]
## Roadmap
The next phase of development will focus on additional financial functions.
## Installation
ex_money can be installed by:
1. Adding `ex_money` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[{:ex_money, "~> 0.0.13"}]
end
```
2. Ensuring `ex_money` is started before your application:
```elixir
def application do
[applications: [:ex_money]]
end
```
