Here's a peek:
/opt/local/lib/ruby/gems/1.8/gems/dm-validations-0.9.3/lib/dm-validations/numeric_validator.rb:17:in `send': undefined method `product_id' for #<Shirt:0x25d9628> (NoMethodError)
from /opt/local/lib/ruby/gems/1.8/gems/dm-validations-0.9.3/lib/dm-validations/numeric_validator.rb:17:in `call'
from /opt/local/lib/ruby/gems/1.8/gems/dm-validations-0.9.3/lib/dm-validations/contextual_validators.rb:48:in `execute'