What is the best way to assign a resource to an attribute?

[stalcottsmith]

I am exploring whether to use this gem for a multi currency accounting system. It is old and presumably no longer developed but I liked the concept of being able to store the value and currency together and ensure that I would not be summing unlike quantities.

I tried using the ActiveRecord integration and setting the amount column to type string on my line_items model.

Then I tried creating with something like:

entry.line_items.create!(amount: '$40145.00')

In the LineItem class, I have:

serialize :amount, Latinum::Resource

This did not work. Amount is nil. I tried overriding amount=(value) and parsing:

  def amount=(value)
    if value.kind_of?(String)
      bank = Latinum::Bank.new(Latinum::Currencies::Global)
      attributes['amount'] = bank.parse(value)
      byebug
    end
  end

At this point the attribute is still nil. I tried setting it a variety of ways. None seemed to work.

What is the recommended way to do this?

[stalcottsmith]

I found that I was able to do this with write_attribute(). But I would still like to know the canonical or expected usage.

[ioquatix]

I'm still using this gem and I suppose you could call it maintained, it's just been stable enough that no changes were required.

The following code was implemented for support of serialize:

latinum/lib/latinum/resource.rb

Lines 40 to 57 in acf12ea

	# Load a string representation of a resource.
	# @parameter string [String | Nil] e.g. "5 NZD" or nil.
	# @returns [Resource | Nil] The Resource that represents the parsed string.
	def self.load(string)
	if string
	# Remove any whitespaces
	string = string.strip
	parse(string) unless string.empty?
	end
	end
	# Dump a string representation of a resource.
	# @parameter resource [Resource] The resource to dump.
	# @returns [String | Nil] A string that represents the {Resource}.
	def self.dump(resource)
	resource.to_s if resource
	end

A bank object is only needed for formatting and converting resources, it should not be needed for basic interactions with "I have X units of Y."

For actual values, you need to write:

entry.line_items.create!(amount: '40145.00 USD')

If you want rich input, you do need to use the bank configured with the currencies you want. Otherwise, how can you tell $5 is USD, NZD, AUD, etc.

In order to use the bank object, you should configure it in the controller to map the user input before creating the model with attribute values.

e.g.

params['amount'] = bank.parse(params['amount'])

I can write updated documentation with working examples if that helps.

[ioquatix]

I wonder if we should make Resource.load also check for Resource instances and return it directly to support the above use case if it isn't already handled by AR.

[ioquatix]

Okay, so with a few minor changes, we can make this work a bit more nicely:

Loading development environment (Rails 7.1.1)
irb(main):001> t = T.new
=> #<T:0x00007f2fb8b88ff0 id: nil, data: nil, amount: nil, created_at: nil, updated_at: nil>
irb(main):002> t.amount = "$5 USD"
=> "$5 USD"
irb(main):003> t.amount
=> #<Latinum::Resource "5.0 USD">

The code is:

require 'latinum/currencies/global'

class T < ApplicationRecord
  BANK = Latinum::Bank.new.tap do |bank|
    bank.import(Latinum::Currencies::Global)
  end
  
  serialize :amount, coder: BANK
end

This uses the bank as a coder, which accepts a wider range of inputs. However, if there is ambiguity, it is resolved according to the bank's internal priority, e.g. $ is used by several currencies.

[ioquatix]

I've released v1.8.0 with the required changes and added documentation: https://ioquatix.github.io/latinum/guides/activerecord-integration/index.html