SendGrid: Fix multiple recipients with only merge_global_data

In SendGrid backend, support non-batch template send to multiple
recipients when `merge_global_data` is set without `merge_data`.
Regression introduced in v6.0.

Fixes #179

CHANGELOG.rst

@@ -25,6 +25,19 @@ Release history
 ^^^^^^^^^^^^^^^
     ..  This extra heading level keeps the ToC from becoming unmanageably long
 
+vNext
+-----
+
+*Unreleased changes on master*
+
+Fixes
+~~~~~
+
+* **SendGrid:** Allow non-batch template send to multiple recipients when
+  `merge_global_data` is set without `merge_data`. (Broken in v6.0. Thanks to
+  `@vgrebenschikov`_ for the bug report.)
+
+
 v7.0
 ----
 
@@ -1021,4 +1034,5 @@ Features
 .. _@sebbacon: https://github.com/sebbacon
 .. _@Thorbenl: https://github.com/Thorbenl
 .. _@varche1: https://github.com/varche1
+.. _@vgrebenschikov: https://github.com/vgrebenschikov
 .. _@yourcelf: https://github.com/yourcelf

anymail/backends/sendgrid.py

@@ -133,8 +133,9 @@ class SendGridPayload(RequestsPayload):
         if self.merge_data or self.merge_global_data:
             # Always build dynamic_template_data first,
             # then convert it to legacy template format if needed
+            only_global_merge_data = self.merge_global_data and not self.merge_data
             for personalization in self.data["personalizations"]:
-                assert len(personalization["to"]) == 1
+                assert len(personalization["to"]) == 1 or only_global_merge_data
                 recipient_email = personalization["to"][0]["email"]
                 dynamic_template_data = self.merge_global_data.copy()
                 dynamic_template_data.update(self.merge_data.get(recipient_email, {}))

tests/test_sendgrid_backend.py

@@ -493,6 +493,20 @@ class SendGridBackendAnymailFeatureTests(SendGridBackendMockAPITestCase):
              'custom_args': {'anymail_id': 'mocked-uuid-2'},
              'substitutions': {"<%test%>": "data"}}])
 
+    def test_merge_data_global_only(self):
+        # a template with only global data can be used to send the same message
+        # to multiple recipients (non-batch)
+        self.message.template_id = "d-5a963add2ec84305813ff860db277d7a"
+        self.message.merge_global_data = {"test": "data"}
+        self.message.to = ["one@example.com", "two@example.com"]
+        self.message.send()
+
+        data = self.get_api_call_json()
+        self.assertEqual(data['personalizations'], [
+            {'to': [{'email': 'one@example.com'}, {'email': 'two@example.com'}],  # not batch
+             'custom_args': {'anymail_id': 'mocked-uuid-1'},
+             'dynamic_template_data': {"test": "data"}}])
+
     def test_legacy_merge_data(self):
         # unless a new "dynamic template" is specified, Anymail assumes the legacy
         # "substitutions" format for merge data