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

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)