added get-method to fetch exactly one document from the collection. catching pymongo's ObjectId-errors and raising mongoengine's ValidationError instead.

mongoengine/base.py

@@ -6,7 +6,6 @@ import pymongo
 class ValidationError(Exception):
     pass
 
-
 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.
@@ -76,7 +75,10 @@ class ObjectIdField(BaseField):
 
     def to_mongo(self, value):
         if not isinstance(value, pymongo.objectid.ObjectId):
-            return pymongo.objectid.ObjectId(str(value))
+            try:
+                return pymongo.objectid.ObjectId(str(value))
+            except Exception, e:
+                raise ValidationError(e.message)
         return value
 
     def prepare_query_value(self, value):

mongoengine/queryset.py

@@ -10,6 +10,10 @@ __all__ = ['queryset_manager', 'Q', 'InvalidQueryError',
 # The maximum number of items to display in a QuerySet.__repr__
 REPR_OUTPUT_SIZE = 20
 
+class DoesNotExist(Exception):
+	pass
+
+
 class MultipleObjectsReturned(Exception):
     pass
 
@@ -314,6 +318,16 @@ class QuerySet(object):
         else:
             raise MultipleObjectsReturned(u'%d items returned, instead of 1' % cnt)
 
+    def get(self, **kwargs):
+        dataset = self.filter(**kwargs)
+        cnt = dataset.count()
+        if cnt == 1:
+            return dataset.first()
+        elif cnt > 1:
+            raise MultipleObjectsReturned(u'%d items returned, instead of 1' % cnt)
+        else:
+            raise DoesNotExist('Document not found')
+
     def first(self):
         """Retrieve the first object matching the query.
         """