diff --git a/odoo/addons/test_new_api/tests/test_onchange.py b/odoo/addons/test_new_api/tests/test_onchange.py
index 1bfdee908a7..03cad7f4009 100644
--- a/odoo/addons/test_new_api/tests/test_onchange.py
+++ b/odoo/addons/test_new_api/tests/test_onchange.py
@@ -1,4 +1,8 @@
 # -*- coding: utf-8 -*-
+try:
+    from unittest.mock import patch
+except ImportError:
+    from mock import patch
 
 from odoo.tests import common
 
@@ -465,3 +469,32 @@ class TestOnChange(common.TransactionCase):
         result = Message.onchange(value, ['message', 'message_name', 'message_currency'], field_onchange)
 
         self.assertEqual(result['value'], onchange_result)
+
+    def test_onchange_many2one_one2many(self):
+        """ Setting a many2one field should not read the inverse one2many. """
+        discussion = self.env.ref('test_new_api.discussion_0')
+        field_onchange = self.Message._onchange_spec()
+        self.assertEqual(field_onchange.get('discussion'), '1')
+
+        values = {
+            'discussion': discussion.id,
+            'name': "[%s] %s" % ('', self.env.user.name),
+            'body': False,
+            'author': self.env.uid,
+            'size': 0,
+        }
+
+        called = [False]
+        orig_read = type(discussion).read
+
+        def mock_read(self, fields=None, load='_classic_read'):
+            if discussion in self and 'messages' in (fields or ()):
+                called[0] = True
+            return orig_read(self, fields, load)
+
+        # changing 'discussion' on message should not read 'messages' on discussion
+        with patch.object(type(discussion), 'read', mock_read, create=True):
+            self.env.cache.invalidate()
+            self.Message.onchange(values, 'discussion', field_onchange)
+
+        self.assertFalse(called[0], "discussion.messages has been read")
diff --git a/odoo/api.py b/odoo/api.py
index 21095854a70..903e1187329 100644
--- a/odoo/api.py
+++ b/odoo/api.py
@@ -982,6 +982,12 @@ class Cache(object):
         value = self._data[field][record.id].get(key, SpecialValue(None))
         return default if isinstance(value, SpecialValue) else value
 
+    def get_special(self, record, field, default=None):
+        """ Return the special value of ``field`` for ``record``. """
+        key = field.cache_key(record)
+        value = self._data[field][record.id].get(key)
+        return value.get if isinstance(value, SpecialValue) else default
+
     def set_special(self, record, field, getter):
         """ Set the value of ``field`` for ``record`` to return ``getter()``. """
         key = field.cache_key(record)
diff --git a/odoo/fields.py b/odoo/fields.py
index 58f7dc67dcb..91cde531649 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -2019,6 +2019,26 @@ class Many2one(_Relational):
         return super(Many2one, self).convert_to_onchange(value, record, names)
 
 
+class _RelationalMultiUpdate(object):
+    """ A getter to update the value of an x2many field, without reading its
+        value until necessary.
+    """
+    __slots__ = ['record', 'field', 'value']
+
+    def __init__(self, record, field, value):
+        self.record = record
+        self.field = field
+        self.value = value
+
+    def __call__(self):
+        # determine the current field's value, and update it in cache only
+        record, field, value = self.record, self.field, self.value
+        cache = record.env.cache
+        cache.remove(record, field)
+        val = field.convert_to_cache(record[field.name] | value, record, validate=False)
+        cache.set(record, field, val)
+        return val
+
 
 class _RelationalMulti(_Relational):
     """ Abstract class for relational fields *2many. """
@@ -2030,21 +2050,16 @@ class _RelationalMulti(_Relational):
         """ Update the cached value of ``self`` for ``records`` with ``value``. """
         cache = records.env.cache
         for record in records:
-            if cache.contains(record, self):
+            special = cache.get_special(record, self)
+            if isinstance(special, _RelationalMultiUpdate):
+                # include 'value' in the existing _RelationalMultiUpdate; this
+                # avoids reading the field's value (which may be large)
+                special.value |= value
+            elif cache.contains(record, self):
                 val = self.convert_to_cache(record[self.name] | value, record, validate=False)
                 cache.set(record, self, val)
             else:
-                cache.set_special(record, self, self._update_getter(record, value))
-
-    def _update_getter(self, record, value):
-        def getter():
-            # determine the current field's value, and update it in cache only
-            cache = record.env.cache
-            cache.remove(record, self)
-            val = self.convert_to_cache(record[self.name] | value, record, validate=False)
-            cache.set(record, self, val)
-            return val
-        return getter
+                cache.set_special(record, self, _RelationalMultiUpdate(record, self, value))
 
     def convert_to_cache(self, value, record, validate=True):
         # cache format: tuple(ids)