Do not use class value of Reference field if it is not a ModelStorage

When Reference fields store partial reference, the value is the stored string
instead of an instance. So the class of the value is then str which can not be
used for domain validation.

Closes #11954

trytond/trytond/model/modelstorage.py

@@ -1207,7 +1207,9 @@ class ModelStorage(Model):
                     Relation = field.get_target()
                 elif field._type == 'reference':
                     value = getattr(record, field.name)
-                    Relation = value.__class__ if value else None
+                    Relation = (
+                        value.__class__ if isinstance(value, ModelStorage)
+                        else None)
                 else:
                     Relation = cls
                 return Relation

trytond/trytond/tests/test_field_reference.py

@@ -510,6 +510,28 @@ class FieldReferenceTestCase(unittest.TestCase):
                     }])
         self.assertEqual(reference.reference, None)
 
+    @with_transaction()
+    def test_domain_no_id_value(self):
+        "Test reference with domain and no id"
+        pool = Pool()
+        Reference = pool.get('test.reference_domainvalidation')
+
+        reference, = Reference.create([{
+                    'reference': 'test.reference.target,',
+                    }])
+        self.assertEqual(reference.reference, 'test.reference.target,')
+
+    @with_transaction()
+    def test_domain_negative_id_value(self):
+        "Test reference with domain and negative id"
+        pool = Pool()
+        Reference = pool.get('test.reference_domainvalidation')
+
+        reference, = Reference.create([{
+                    'reference': 'test.reference.target,-1',
+                    }])
+        self.assertEqual(reference.reference, 'test.reference.target,-1')
+
     @with_transaction()
     def test_domain_different_targets(self):
         "Test reference with domain and different targets"
@@ -544,6 +566,28 @@ class FieldReferenceTestCase(unittest.TestCase):
                     }])
         self.assertEqual(reference.reference, target)
 
+    @with_transaction()
+    def test_domain_pyson_no_id_value(self):
+        "Test reference with pyson domain and no id"
+        pool = Pool()
+        Reference = pool.get('test.reference_domainvalidation_pyson')
+
+        reference, = Reference.create([{
+                    'reference': 'test.reference.target,',
+                    }])
+        self.assertEqual(reference.reference, 'test.reference.target,')
+
+    @with_transaction()
+    def test_domain_pyson_negative_id_value(self):
+        "Test reference with pyson domain and negative id"
+        pool = Pool()
+        Reference = pool.get('test.reference_domainvalidation_pyson')
+
+        reference, = Reference.create([{
+                    'reference': 'test.reference.target,-1',
+                    }])
+        self.assertEqual(reference.reference, 'test.reference.target,-1')
+
     @with_transaction()
     def test_domain_pyson_different_targets(self):
         "Test reference with pyson domain and different targets"