diff --git a/registrasion/controllers/credit_note.py b/registrasion/controllers/credit_note.py
index e1f0ed2b..182c10e9 100644
--- a/registrasion/controllers/credit_note.py
+++ b/registrasion/controllers/credit_note.py
@@ -2,8 +2,12 @@ from django.db import transaction
 
 from registrasion.models import commerce
 
+from for_id import ForId
 
-class CreditNoteController(object):
+
+class CreditNoteController(ForId, object):
+
+    __MODEL__ = commerce.CreditNote
 
     def __init__(self, credit_note):
         self.credit_note = credit_note
diff --git a/registrasion/controllers/for_id.py b/registrasion/controllers/for_id.py
new file mode 100644
index 00000000..3748b151
--- /dev/null
+++ b/registrasion/controllers/for_id.py
@@ -0,0 +1,24 @@
+from django.core.exceptions import ObjectDoesNotExist
+from django.http import Http404
+
+
+class ForId(object):
+    ''' Mixin class that gives you new classmethods: for_id for_id_or_404.
+    These let you retrieve an instance of the class by specifying the model ID.
+
+    Your subclass must define __MODEL__ as a class attribute. This will be the
+    model class that we wrap. There must also be a constructor that takes a
+    single argument: the instance of the model that we are controlling. '''
+
+    @classmethod
+    def for_id(cls, id_):
+        id_ = int(id_)
+        obj = cls.__MODEL__.objects.get(pk=id_)
+        return cls(obj)
+
+    @classmethod
+    def for_id_or_404(cls, id_):
+        try:
+            return cls.for_id(id_)
+        except ObjectDoesNotExist:
+            return Http404
diff --git a/registrasion/controllers/invoice.py b/registrasion/controllers/invoice.py
index fff97e8d..d2b6bf3f 100644
--- a/registrasion/controllers/invoice.py
+++ b/registrasion/controllers/invoice.py
@@ -11,9 +11,11 @@ from registrasion.models import people
 
 from cart import CartController
 from credit_note import CreditNoteController
+from for_id import ForId
 
+class InvoiceController(ForId, object):
 
-class InvoiceController(object):
+    __MODEL__ = commerce.Invoice
 
     def __init__(self, invoice):
         self.invoice = invoice
diff --git a/registrasion/tests/test_invoice.py b/registrasion/tests/test_invoice.py
index 3a655bb1..8e1c7ac0 100644
--- a/registrasion/tests/test_invoice.py
+++ b/registrasion/tests/test_invoice.py
@@ -53,6 +53,20 @@ class InvoiceTestCase(RegistrationCartTestCase):
             self.PROD_1.price + self.PROD_2.price,
             invoice_2.invoice.value)
 
+    def test_invoice_controller_for_id_works(self):
+        current_cart = TestingCartController.for_user(self.USER_1)
+        current_cart.add_to_cart(self.PROD_1, 1)
+
+        invoice = TestingInvoiceController.for_cart(current_cart.cart)
+
+        id_ = invoice.invoice.id
+
+        invoice1 = TestingInvoiceController.for_id(id_)
+        invoice2 = TestingInvoiceController.for_id(str(id_))
+
+        self.assertEqual(invoice.invoice, invoice1.invoice)
+        self.assertEqual(invoice.invoice, invoice2.invoice)
+
     def test_create_invoice_fails_if_cart_invalid(self):
         self.make_ceiling("Limit ceiling", limit=1)
         self.set_time(datetime.datetime(2015, 01, 01, tzinfo=UTC))