scopes related question

Hello,
Have a question related scope definition. I need to define a scope
based on related model attribute values like this.

class Product < ActiveRecord::Base
   belongs_to :currency

scope :with_currency, lambda { |currency|
    unless currency.to_s.upcase == 'ALL'

# here I need to define a scope that would test for currency.char_code
to match a passed value
# something like.. This is where I need help
      where("currency.char_code = ?", currency.to_s.upcase)
    end
  }

end

class Currency < ActiveRecord::Base
attr_accessor : char_code
end

class LineItem < ActiveRecord::Base
    belongs_to :product

   scope :with_currency, lambda { |currency|
    unless currency.to_s.upcase == 'ALL'

# here is another scope where I need help..
      where("product.currency.char_code = ?", currency.to_s.upcase)
    end
  }

end

Thanks for suggestions. Still learning the scoping magic.

figured out the answer to this problem. I needed to use :joins to
include associated models in scopes.