diff --git a/openerp/addons/base/tests/test_expression.py b/openerp/addons/base/tests/test_expression.py
index 2bfd8ade1c9..a8b69ac8c06 100644
--- a/openerp/addons/base/tests/test_expression.py
+++ b/openerp/addons/base/tests/test_expression.py
@@ -126,9 +126,10 @@ class test_expression(common.TransactionCase):
         # Get test columns
         partner_state_id_col = partner_obj._columns.get('state_id')  # many2one on res.partner to res.country.state
         partner_parent_id_col = partner_obj._columns.get('parent_id')  # many2one on res.partner to res.partner
+        state_country_id_col = state_obj._columns.get('country_id')  # many2one on res.country.state on res.country
         partner_child_ids_col = partner_obj._columns.get('child_ids')  # one2many on res.partner to res.partner
         partner_bank_ids_col = partner_obj._columns.get('bank_ids')  # one2many on res.partner to res.partner.bank
-        state_country_id_col = state_obj._columns.get('country_id')  # many2one on res.country.state on res.country
+        category_id_col = partner_obj._columns.get('category_id')  # many2many on res.partner to res.partner.category
 
         # Get the first bank account type to be able to create a res.partner.bank
         bank_type = bank_obj._bank_type_get(cr, uid)[0]
@@ -146,6 +147,14 @@ class test_expression(common.TransactionCase):
         b_ab = bank_obj.create(cr, uid, {'name': '__bank_test_b', 'state': bank_type[0], 'partner_id': p_ab, 'acc_number': '5678'})
         b_ba = bank_obj.create(cr, uid, {'name': '__bank_test_b', 'state': bank_type[0], 'partner_id': p_ba, 'acc_number': '9876'})
 
+        # ----------------------------------------
+        # Test1: basics about the attribute
+        # ----------------------------------------
+
+        category_id_col._auto_join = True
+        self.assertRaises(NotImplementedError, partner_obj.search, cr, uid, [('category_id.name', '=', 'foo')])
+        category_id_col._auto_join = False
+
         # ----------------------------------------
         # Test1: one2many
         # ----------------------------------------
diff --git a/openerp/osv/expression.py b/openerp/osv/expression.py
index c4c7f4400ea..383e70dccfa 100644
--- a/openerp/osv/expression.py
+++ b/openerp/osv/expression.py
@@ -800,8 +800,7 @@ class expression(object):
                 leafs_to_stack.append(self.create_substitution_leaf(leaf, (field_path[1], operator, right), relational_table))
 
             elif len(field_path) > 1 and field._auto_join:
-                assert False, \
-                    '_auto_join attribute on something else than a many2one or one2many is currently not supported'
+                raise NotImplementedError('_auto_join attribute not supported on many2many field %s' % (left))
 
             elif len(field_path) > 1 and field._type == 'many2one':
                 right_ids = relational_table.search(cr, uid, [(field_path[1], operator, right)], context=context)