SendGrid: set to to from in batch send.

Set ignored (but required-valid) `to` field to
the from_email for batch send with merge_data.
See #14 (comment)

Author: medmunds

anymail/backends/sendgrid.py

@@ -96,9 +96,14 @@ def serialize_data(self):
 
         self.build_merge_data()
         if self.merge_data is not None:
-            # Must *also* set smtpapi 'to' field so SG does batch send
-            # (else all recipients would see each other's emails)
+            # Move the 'to' recipients to smtpapi, so SG does batch send
+            # (else all recipients would see each other's emails).
+            # Regular 'to' must still be a valid email (even though "ignored")...
+            # we use the from_email as recommended by SG support
+            # (See https://github.com/anymail/django-anymail/pull/14#issuecomment-220231250)
             self.smtpapi['to'] = [email.address for email in self.to_list]
+            self.data['to'] = [self.data['from']]
+            self.data['toname'] = [self.data.get('fromname', " ")]
 
         # Serialize x-smtpapi to json:
         if len(self.smtpapi) > 0:

tests/test_sendgrid_backend.py

@@ -413,6 +413,7 @@ def test_template_id(self):
         })
 
     def test_merge_data(self):
+        self.message.from_email = '[email protected]'
         self.message.to = ['[email protected]', 'Bob <[email protected]>']
         # SendGrid template_id is not required to use merge.
         # You can just supply template content as the message (e.g.):
@@ -431,10 +432,12 @@ def test_merge_data(self):
 
         data = self.get_api_call_data()
         smtpapi = self.get_smtpapi()
-        # For batch send, must set both to+toname *and* smtpapi['to']:
-        self.assertEqual(data['toname'], [' ', 'Bob'])
-        self.assertEqual(data['to'], ['[email protected]', '[email protected]'])
+        # For batch send, smtpapi['to'] gets real recipient list;
+        # normal 'to' is not used (but must be valid, so we substitute the from_email):
+        self.assertEqual(data['to'], ['[email protected]'])
+        self.assertEqual(data['toname'], [' '])  # empty string if no name in from_email
         self.assertEqual(smtpapi['to'], ['[email protected]', 'Bob <[email protected]>'])
+        # smtpapi['sub'] values should be in to-list order:
         self.assertEqual(smtpapi['sub'], {
             ':name': ["Alice", "Bob"],
             ':group': ["Developers", ":group"],  # missing value gets replaced with var name...