new save() method updates only dirty fields. fixes issue #18

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):