Modelling money

Rails
1

hi

i’m trying to write an app to keep track of expenses and invoices etc.
for my small business. it’s mainly an excuse to learn ROR but in doing
so i’ve hit upon a problem: money. I need to store monetary values in
the database (MySQL) which I believe you would normally use decimal
type for. My understanding is that rails is going to support that in
the next version with Active Record but doesn’t currently. Also using
floats is to be avoided due to rounding errors.

So what does one do in the mean time? I found a money gem
http://rubyforge.org/projects/money/ with no real documentation (I’m
looking for worked examples to learn from). or a do-it-yourself method
ActiveRecord::Aggregations::ClassMethods

This seems like such a simple problem I can’t believe I’ve spent so
long trying to solve it - can somebody please tell me an easy way to do
this?

thanks, mark

2

I haven’t done it yet, but I just plan on writing a method which will
convert a string (eg, ‘5.28’) to the number 520, and then another to
convert back to ‘5.28’. That makes a nice easy way to store money as
integers.
Hope this helps.
-Nathan

3

*sorry, that should be to the number 528.

4

Hi !

2006/9/15, creativetags [email protected]:

So what does one do in the mean time? I found a money gem
http://rubyforge.org/projects/money/ with no real documentation (I’m
looking for worked examples to learn from). or a do-it-yourself method
ActiveRecord::Aggregations::ClassMethods

Use the Money gem. Then in your model, you simply compose yourself:

class LineItem < ActiveRecord::Base
composed_of :unit_price, :class_name => ‘Money’, :mapping =>
[%w(unit_price_cents cents), %w(unit_price_currency currency)]
end

Hope that helps !

François Beausoleil
http://blog.teksol.info/
http://piston.rubyforge.org/

5

thanks for all the suggestions. I guess the problem doesn’t lie in
outputting a stored integer to the view as thats easy to format as Roy
Tinker suggested. How do you get a value a user inputs into a form
field (e.g. £10.23) into a balance variable in the object model?

You could validate in the form (and maybe do some formatting to get it
to 1023 or something) but this seems risky - what if javascript is
disabled?

Alternatively you get the form back to the controller and i’m used to
doing:
@invoice = Invoice.find(params[:id])
but if you’re using the Money gem that won’t work will it? you need to
create a new Money object and pass that into the balance field of
@invoice

I really want a simple way to do this in the controller (or model)

6

I haven’t done it yet, but I just plan on writing a method which will
convert a string (eg, ‘5.28’) to the number 520, and then another to
convert back to ‘5.28’. That makes a nice easy way to store money as
integers.

But you also lost the ability to do math directly in the database (at
least for db’s that distinguish b/n strings and integers)…

I think until Rails supports decimal you should just use the base unit
(for US - store it in cents) and convert in the views as needed.

7

creativetags wrote:

I need to store monetary values in the database (MySQL) which I believe you
would normally use decimal type for. My understanding is that rails is going
to support that in the next version with Active Record but doesn’t currently.

You can use edge Rails. I also needed decimal support but my app had
tests that
didn’t pass under edge Rails, so I backported decimal support to current
stable
release (1.1.6). If you’re interested in using the backport, I can send
you the
patch.

–
Sava C.

8

I’m using the dollars and cents plugin.

http://blog.codahale.com/2006/05/18/dollars_and_cents-a-rails-plugin/

You make your db column an int, and the plugin handles the conversion
for you, invisibly.

On 9/15/06, creativetags [email protected] wrote:

–
To get the programming fast enough so that it’s not a boat anchor
[…] is to program less. There are lots of things we can program, but
just because we can doesn’t mean we should. – Ward Cunningham