[FIX] models: in onchange, false changes where detected in many2many fields

This was due to secondary fields loaded from database in 'onchange' mode.  In
that mode, the secondary fields were marked 'dirty', and therefore returned by
the method `onchange`.  The fix consists in loading those secondary fields in
cache before processing the onchanges.

This incidentally fixes a test on method `onchange`: in a one2many field, some
dirty fields were unexpectedly returned in the result.  This was due to those
fields being loaded while processing onchanges.

openerp/addons/test_new_api/tests/test_onchange.py

@@ -146,8 +146,6 @@ class TestOnChange(common.TransactionCase):
             }),
             (1, message.id, {
                 'name': "[%s] %s" % ("Foo", USER.name),
-                # Note: size is computed because it was not provided beforehand
-                'size': len(BODY),
             }),
         ])
 

openerp/models.py

@@ -5787,6 +5787,11 @@ class BaseModel(object):
             # attach `self` with a different context (for cache consistency)
             record._origin = self.with_context(__onchange=True)
 
+        # load fields on secondary records, to avoid false changes
+        with env.do_in_onchange():
+            for field_seq in secondary:
+                record.mapped(field_seq)
+
         # determine which field(s) should be triggered an onchange
         todo = list(names) or list(values)
         done = set()