undefined local variable or method `invoices' for Provider:Class

I'm pretty new to Rails, and I think my app has progressed quicker than
my knowledge of Rails/Ruby has ...

I have the following models:

[ -- Begin -- ]
class Provider < ActiveRecord::Base
  has_many :invoices
  has_one :user

  def self.get_invoices
    invoices.find(:all)
  end
end

class User < ActiveRecord::Base
  belongs_to :Provider
  belongs_to :Consumer

  def invoices_raised
    Provider.get_invoices
  end

   ...

end
[ -- End -- ]

and when I call User.invoices_raised I get this error:

undefined local variable or method `invoices' for Provider:Class

Now I thought that having the "has_many :invoices" line would
automatically make the connection and let me access "invoices" like
that, but obviously I'm wrong. Anyone care to correct me?

Cheers,

-S