Fixed #11929 -- Make sure YAML output order is helpful

By adopting Python 3.6+, this was already ensured, but this test makes
sure we'll have no regression.

tests/serializers/test_yaml.py

@@ -85,29 +85,29 @@ def test_dumpdata_pyyaml_error_message(self):
 @unittest.skipUnless(HAS_YAML, "No yaml library detected")
 class YamlSerializerTestCase(SerializersTestBase, TestCase):
     serializer_name = "yaml"
-    fwd_ref_str = """- fields:
+    fwd_ref_str = """- model: serializers.article
+  pk: 1
+  fields:
     headline: Forward references pose no problem
     pub_date: 2006-06-16 15:00:00
     categories: [1]
     author: 1
+- model: serializers.category
   pk: 1
-  model: serializers.article
-- fields:
+  fields:
     name: Reference
+- model: serializers.author
   pk: 1
-  model: serializers.category
-- fields:
-    name: Agnes
-  pk: 1
-  model: serializers.author"""
+  fields:
+    name: Agnes"""
 
-    pkless_str = """- fields:
-    name: Reference
+    pkless_str = """- model: serializers.category
   pk: null
-  model: serializers.category
-- fields:
-    name: Non-fiction
-  model: serializers.category"""
+  fields:
+    name: Reference
+- model: serializers.category
+  fields:
+    name: Non-fiction"""
 
     mapping_ordering_str = """- model: serializers.article
   pk: %(article_pk)s
@@ -159,22 +159,48 @@ def test_yaml_deserializer_exception(self):
             for obj in serializers.deserialize("yaml", "{"):
                 pass
 
+    def test_yaml_output_ordering(self):
+        """Output is in the expected order of model, pk, fields."""
+        from .models import Article
+        serial_str = serializers.serialize(self.serializer_name, Article.objects.all())
+        self.assertEqual(serial_str, """- model: serializers.article
+  pk: 1
+  fields:
+    author: 2
+    headline: Poker has no place on ESPN
+    pub_date: 2006-06-16 11:00:00
+    categories:
+    - 3
+    - 1
+    meta_data: []
+- model: serializers.article
+  pk: 2
+  fields:
+    author: 1
+    headline: Time to reform copyright
+    pub_date: 2006-06-16 13:00:11.000345
+    categories:
+    - 2
+    - 3
+    meta_data: []
+""")
+
 
 @unittest.skipUnless(HAS_YAML, "No yaml library detected")
 class YamlSerializerTransactionTestCase(SerializersTransactionTestBase, TransactionTestCase):
     serializer_name = "yaml"
-    fwd_ref_str = """- fields:
+    fwd_ref_str = """- model: serializers.article
+  pk: 1
+  fields:
     headline: Forward references pose no problem
     pub_date: 2006-06-16 15:00:00
     categories: [1]
     author: 1
+- model: serializers.category
   pk: 1
-  model: serializers.article
-- fields:
+  fields:
     name: Reference
+- model: serializers.author
   pk: 1
-  model: serializers.category
-- fields:
-    name: Agnes
-  pk: 1
-  model: serializers.author"""
+  fields:
+    name: Agnes"""