Do not compute record name for negative id

With ba6b8ce0e692 the record name of default values are also computed for
one2one and reference but reference fields can have a default value with only
the model part by using a negative id. In this case we should not compute a
record name.

trytond/trytond/model/model.py

@@ -170,10 +170,11 @@ class Model(URLMixin, PoolBase, metaclass=ModelMeta):
                     if field._type == 'reference':
                         target_name, target_id = value.split(',')
                         Target = pool.get(target_name)
+                        target_id = int(target_id)
                     else:
                         Target = field.get_target()
                         target_id = value
-                    if 'rec_name' in Target._fields:
+                    if 'rec_name' in Target._fields and target_id >= 0:
                         target = Target(target_id)
                         values.setdefault(
                             field_name + '.', {})['rec_name'] = target.rec_name

trytond/trytond/tests/model.py

@@ -69,6 +69,9 @@ class ModelDefault(Model):
     name = fields.Char("Name")
     description = fields.Char("Description")
     target = fields.Many2One('test.model', "Target")
+    reference = fields.Reference("Reference", [
+            ('test.model', "Model"),
+            ])
 
     @classmethod
     def default_description(cls):

trytond/trytond/tests/test_model.py

@@ -374,6 +374,18 @@ class ModelTestCase(TestCase):
                         },
                     })
 
+    @with_transaction()
+    def test_default_get_with_model_reference_rec_name(self):
+        "Test default_get with model reference and rec_name"
+        pool = Pool()
+        Model = pool.get('test.model.default')
+
+        with Transaction().set_context(default_reference='test.model,-1'):
+            self.assertEqual(
+                Model.default_get(['reference'], with_rec_name=True), {
+                    'reference': 'test.model,-1',
+                    })
+
     @with_transaction()
     def test_default_get_without_rec_name(self):
         "Test default_get without rec_name"