diff --git a/mongoengine/base.py b/mongoengine/base.py
index 8c038c8c..b4920eec 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -15,13 +15,14 @@ class BaseField(object):
     _index_with_types = True
     
     def __init__(self, name=None, required=False, default=None, unique=False,
-                 unique_with=None, primary_key=False):
+                 unique_with=None, primary_key=False, modified=False):
         self.name = name if not primary_key else '_id'
         self.required = required or primary_key
         self.default = default
         self.unique = bool(unique or unique_with)
         self.unique_with = unique_with
         self.primary_key = primary_key
+        self.modified = modified
 
     def __get__(self, instance, owner):
         """Descriptor for retrieving a value from a field in a document. Do 
@@ -44,6 +45,7 @@ class BaseField(object):
         """Descriptor for assigning a value to a field in a document.
         """
         instance._data[self.name] = value
+        self.modified = True
 
     def to_python(self, value):
         """Convert a MongoDB-compatible type to a Python type.
@@ -252,8 +254,11 @@ class BaseDocument(object):
 
     def __init__(self, **values):
         self._data = {}
+        
+        modified = 'id' in values.keys()
         # Assign initial values to instance
         for attr_name, attr_value in self._fields.items():
+            attr_value.modified = modified
             if attr_name in values:
                 setattr(self, attr_name, values.pop(attr_name))
             else:
@@ -381,9 +386,9 @@ class BaseDocument(object):
                 # that has been queried to return this SON
                 return None
             cls = subclasses[class_name]
-
+        
         for field_name, field in cls._fields.items():
             if field.name in data:
                 data[field_name] = field.to_python(data[field.name])
-
-        return cls(**data)
+        
+        return cls(**data)
\ No newline at end of file
diff --git a/mongoengine/document.py b/mongoengine/document.py
index 62f9ecce..eec31e09 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -1,8 +1,9 @@
 from base import (DocumentMetaclass, TopLevelDocumentMetaclass, BaseDocument,
                   ValidationError)
-from queryset import OperationError
+from queryset import OperationError, QuerySet
 from connection import _get_db
 
+
 import pymongo
 
 
@@ -75,12 +76,30 @@ class Document(BaseDocument):
             if force_insert:
                 object_id = collection.insert(doc, safe=safe)
             else:
-                object_id = collection.save(doc, safe=safe)
+                if getattr(self, 'id', None) == None:
+                    # new document
+                    object_id = collection.save(doc, safe=safe)
+                else:
+                    # update document
+                    modified_fields = map(lambda obj: obj[0], filter(lambda obj: obj[1].modified, self._fields.items()))
+                    modified_doc = dict(filter(lambda k: k[0] in modified_fields, doc.items()))                
+                    try:
+                        id_field = self._meta['id_field']
+                        idObj = self._fields[id_field].to_mongo(self['id'])
+                        collection.update({'_id': idObj}, {'$set': modified_doc}, safe=safe)
+                    except pymongo.errors.OperationFailure, err:
+                        if str(err) == 'multi not coded yet':
+                            raise OperationError('update() method requires MongoDB 1.1.3+')
+                        raise OperationError('Update failed (%s)' % str(err))
+                    object_id = self['id']
+
+            for field in self._fields.values(): field.modified = False
         except pymongo.errors.OperationFailure, err:
             message = 'Could not save document (%s)'
             if 'duplicate key' in str(err):
                 message = 'Tried to save duplicate unique keys (%s)'
             raise OperationError(message % str(err))
+
         id_field = self._meta['id_field']
         self[id_field] = self._fields[id_field].to_python(object_id)
 
@@ -106,6 +125,7 @@ class Document(BaseDocument):
         obj = self.__class__.objects(**{id_field: self[id_field]}).first()
         for field in self._fields:
             setattr(self, field, obj[field])
+            obj.modified = False
 
     @classmethod
     def drop_collection(cls):
diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index beb8ae00..90a3b3d2 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -71,6 +71,7 @@ class FloatField(BaseField):
         return float(value)
 
     def validate(self, value):
+        if isinstance(value, int): value = float(value)
         assert isinstance(value, float)
 
         if self.min_value is not None and value < self.min_value:
diff --git a/mongoengine/queryset.py b/mongoengine/queryset.py
index 69bec002..e71f0598 100644
--- a/mongoengine/queryset.py
+++ b/mongoengine/queryset.py
@@ -316,15 +316,17 @@ class QuerySet(object):
         elif cnt == 1:
             return dataset.first()
         else:
-            raise MultipleObjectsReturned(u'%d items returned, instead of 1' % cnt)
+            raise MultipleObjectsReturned(u'%d items returned, expected exactly one' % cnt)
 
     def get(self, **kwargs):
+        """Retreive exactly one document. Raise DoesNotExist if it's not found.
+        """
         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)
+            raise MultipleObjectsReturned(u'%d items returned, expected exactly one' % cnt)
         else:
             raise DoesNotExist('Document not found')