Postmark: Fix API error with template but no merge data

Fixes #223

CHANGELOG.rst

@@ -25,6 +25,18 @@ Release history
 ^^^^^^^^^^^^^^^
     ..  This extra heading level keeps the ToC from becoming unmanageably long
 
+vNext
+-----
+
+*unreleased changes*
+
+Fixes
+~~~~~
+
+* **Postmark:** Fix Postmark API error when sending with a template that doesn't
+  require any merge data. (Thanks to `@Tobeyforce`_ for reporting it.)
+
+
 v8.2
 -----
 
@@ -1225,8 +1237,9 @@ Features
 .. _@sebbacon: https://github.com/sebbacon
 .. _@slinkymanbyday: https://github.com/slinkymanbyday
 .. _@swrobel: https://github.com/swrobel
-.. _@Thorbenl: https://github.com/Thorbenl
 .. _@tcourtqtm: https://github.com/tcourtqtm
+.. _@Thorbenl: https://github.com/Thorbenl
+.. _@Tobeyforce: https://github.com/Tobeyforce
 .. _@varche1: https://github.com/varche1
 .. _@vgrebenschikov: https://github.com/vgrebenschikov
 .. _@yourcelf: https://github.com/yourcelf

anymail/backends/postmark.py

@@ -301,6 +301,10 @@ class PostmarkPayload(RequestsPayload):
         except ValueError:
             self.data["TemplateAlias"] = template_id
 
+        # Postmark requires TemplateModel (empty ok) when TemplateId/TemplateAlias
+        # specified. (This may get overwritten by a real TemplateModel later.)
+        self.data.setdefault("TemplateModel", {})
+
         # Subject, TextBody, and HtmlBody aren't allowed with TemplateId;
         # delete Django default subject and body empty strings:
         for field in ("Subject", "TextBody", "HtmlBody"):

tests/test_postmark_backend.py

@@ -373,7 +373,6 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
             # Omit subject and body (Postmark prohibits them with templates)
             from_email='from@example.com', to=['to@example.com'],
             template_id=1234567,
-            # Postmark doesn't support per-recipient merge_data
             merge_global_data={'name': "Alice", 'group': "Developers"},
         )
         message.send()
@@ -396,6 +395,8 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         self.assert_esp_called('/email/withTemplate/')
         data = self.get_api_call_json()
         self.assertEqual(data['TemplateAlias'], 'welcome-message')
+        # Postmark requires TemplateModel (can be empty) with TemplateId/TemplateAlias
+        self.assertEqual(data['TemplateModel'], {})
 
     _mock_batch_response = json.dumps([{
             "ErrorCode": 0,