Updated docs and added a NotRegistered exception

For handling GenericReferences that reference documents that haven't
been imported.

Closes #170

mongoengine/base.py

@@ -7,16 +7,26 @@ import pymongo
 import pymongo.objectid
 
 
-_document_registry = {}
-
-def get_document(name):
-    return _document_registry[name]
+class NotRegistered(Exception):
+    pass
 
 
 class ValidationError(Exception):
     pass
 
 
+_document_registry = {}
+
+def get_document(name):
+    if name not in _document_registry:
+        raise NotRegistered("""
+            `%s` has not been registered in the document registry.
+            Importing the document class automatically registers it, has it
+            been imported?
+        """.strip() % name)
+    return _document_registry[name]
+
+
 class BaseField(object):
     """A base class for fields in a MongoDB document. Instances of this class
     may be added to subclasses of `Document` to define a document's schema.

mongoengine/fields.py

@@ -522,6 +522,9 @@ class GenericReferenceField(BaseField):
     """A reference to *any* :class:`~mongoengine.document.Document` subclass
     that will be automatically dereferenced on access (lazily).
 
+    note:  Any documents used as a generic reference must be registered in the
+    document registry.  Importing the model will automatically register it.
+
     .. versionadded:: 0.3
     """
 

tests/fields.py

@@ -7,6 +7,7 @@ import gridfs
 
 from mongoengine import *
 from mongoengine.connection import _get_db
+from mongoengine.base import _document_registry, NotRegistered
 
 
 class FieldTest(unittest.TestCase):
@@ -584,6 +585,39 @@ class FieldTest(unittest.TestCase):
         Post.drop_collection()
         User.drop_collection()
 
+    def test_generic_reference_document_not_registered(self):
+        """Ensure dereferencing out of the document registry throws a
+        `NotRegistered` error.
+        """
+        class Link(Document):
+            title = StringField()
+
+        class User(Document):
+            bookmarks = ListField(GenericReferenceField())
+
+        Link.drop_collection()
+        User.drop_collection()
+
+        link_1 = Link(title="Pitchfork")
+        link_1.save()
+
+        user = User(bookmarks=[link_1])
+        user.save()
+
+        # Mimic User and Link definitions being in a different file
+        # and the Link model not being imported in the User file.
+        del(_document_registry["Link"])
+
+        user = User.objects.first()
+        try:
+            user.bookmarks
+            raise AssertionError, "Link was removed from the registry"
+        except NotRegistered:
+            pass
+
+        Link.drop_collection()
+        User.drop_collection()
+
     def test_binary_fields(self):
         """Ensure that binary fields can be stored and retrieved.
         """