diff --git a/openerp/addons/test_new_api/tests/test_onchange.py b/openerp/addons/test_new_api/tests/test_onchange.py
index da28bdf0c1b..e9b7e1f83aa 100644
--- a/openerp/addons/test_new_api/tests/test_onchange.py
+++ b/openerp/addons/test_new_api/tests/test_onchange.py
@@ -146,8 +146,7 @@ class TestOnChange(common.TransactionCase):
             }),
             (1, message.id, {
                 'name': "[%s] %s" % ("Foo", USER.name),
-                'body': BODY,
-                'author': USER.id,
+                # Note: size is computed because it was not provided beforehand
                 'size': len(BODY),
             }),
         ])
diff --git a/openerp/api.py b/openerp/api.py
index 94ed98c1265..8be6c268ea9 100644
--- a/openerp/api.py
+++ b/openerp/api.py
@@ -2,7 +2,7 @@
 ##############################################################################
 #
 #    OpenERP, Open Source Management Solution
-#    Copyright (C) 2013 OpenERP (<http://www.openerp.com>).
+#    Copyright (C) 2013-2014 OpenERP (<http://www.openerp.com>).
 #
 #    This program is free software: you can redistribute it and/or modify
 #    it under the terms of the GNU Affero General Public License as
@@ -717,7 +717,7 @@ class Environment(object):
         self.cache = defaultdict(dict)      # {field: {id: value, ...}, ...}
         self.prefetch = defaultdict(set)    # {model_name: set(id), ...}
         self.computed = defaultdict(set)    # {field: set(id), ...}
-        self.dirty = set()                  # set(record)
+        self.dirty = defaultdict(set)       # {record: set(field_name), ...}
         self.all = envs
         envs.add(self)
         return self
diff --git a/openerp/fields.py b/openerp/fields.py
index 27c0df09a4b..0192fbac6c3 100644
--- a/openerp/fields.py
+++ b/openerp/fields.py
@@ -791,7 +791,7 @@ class Field(object):
             if env.in_onchange:
                 for invf in self.inverse_fields:
                     invf._update(value, record)
-                record._dirty = True
+                record._set_dirty(self.name)
 
             # determine more dependent fields, and invalidate them
             if self.relational:
@@ -1577,13 +1577,14 @@ class _RelationalMulti(_Relational):
 
         # add new and existing records
         for record in value:
-            if not record.id or record._dirty:
-                values = dict((k, v) for k, v in record._cache.iteritems() if k in fnames)
+            if not record.id:
+                values = {k: v for k, v in record._cache.iteritems() if k in fnames}
                 values = record._convert_to_write(values)
-                if not record.id:
-                    result.append((0, 0, values))
-                else:
-                    result.append((1, record.id, values))
+                result.append((0, 0, values))
+            elif record._is_dirty():
+                values = {k: record._cache[k] for k in record._get_dirty() if k in fnames}
+                values = record._convert_to_write(values)
+                result.append((1, record.id, values))
             else:
                 add_existing(record.id)
 
diff --git a/openerp/models.py b/openerp/models.py
index d5fde6c9fe5..33327ef7279 100644
--- a/openerp/models.py
+++ b/openerp/models.py
@@ -5343,22 +5343,24 @@ class BaseModel(object):
         return record
 
     #
-    # Dirty flag, to mark records modified (in draft mode)
+    # Dirty flags, to mark record fields modified (in draft mode)
     #
 
-    @property
-    def _dirty(self):
+    def _is_dirty(self):
         """ Return whether any record in `self` is dirty. """
         dirty = self.env.dirty
         return any(record in dirty for record in self)
 
-    @_dirty.setter
-    def _dirty(self, value):
-        """ Mark the records in `self` as dirty. """
-        if value:
-            map(self.env.dirty.add, self)
-        else:
-            map(self.env.dirty.discard, self)
+    def _get_dirty(self):
+        """ Return the list of field names for which `self` is dirty. """
+        dirty = self.env.dirty
+        return list(dirty.get(self, ()))
+
+    def _set_dirty(self, field_name):
+        """ Mark the records in `self` as dirty for the given `field_name`. """
+        dirty = self.env.dirty
+        for record in self:
+            dirty[record].add(field_name)
 
     #
     # "Dunder" methods
@@ -5760,7 +5762,7 @@ class BaseModel(object):
                     field = self._fields[name]
                     newval = record[name]
                     if field.type in ('one2many', 'many2many'):
-                        if newval != oldval or newval._dirty:
+                        if newval != oldval or newval._is_dirty():
                             # put new value in result
                             result['value'][name] = field.convert_to_write(
                                 newval, record._origin, subfields.get(name),