🐛(backend) make message subject optional

- Add null=True, blank=True to Message and Thread subject fields
- Add comprehensive tests for empty/null/missing subjects
- Update migration 0018 to alter subject field constraints
- Fix #161: allow messages without subject field

Author: sdemagny

src/backend/core/mda/inbound.py

@@ -330,7 +330,7 @@ def deliver_inbound_message(  # pylint: disable=too-many-branches, too-many-stat
                 snippet = "(No snippet available)"  # Absolute fallback
 
             thread = models.Thread.objects.create(
-                subject=parsed_email.get("subject", "(no subject)"),
+                subject=parsed_email.get("subject"),
                 snippet=snippet,
             )
             # Create a thread access for the sender mailbox
@@ -441,7 +441,7 @@ def deliver_inbound_message(  # pylint: disable=too-many-branches, too-many-stat
         message = models.Message.objects.create(
             thread=thread,
             sender=sender_contact,
-            subject=parsed_email.get("subject", "(no subject)"),
+            subject=parsed_email.get("subject"),
             raw_mime=raw_data,
             mime_id=parsed_email.get("messageId", parsed_email.get("message_id"))
             or None,

src/backend/core/migrations/0018_alter_label_options_alter_message_subject_and_more.py

@@ -0,0 +1,27 @@
+# Generated by Django 5.1.8 on 2025-07-01 09:27
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0017_remove_thread_count_draft_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='label',
+            options={'ordering': ['slug'], 'verbose_name': 'label', 'verbose_name_plural': 'labels'},
+        ),
+        migrations.AlterField(
+            model_name='message',
+            name='subject',
+            field=models.CharField(blank=True, null=True, max_length=255, verbose_name='subject'),
+        ),
+        migrations.AlterField(
+            model_name='thread',
+            name='subject',
+            field=models.CharField(blank=True, null=True, max_length=255, verbose_name='subject'),
+        ),
+    ]

src/backend/core/models.py

@@ -325,7 +325,7 @@ def __str__(self):
 class Thread(BaseModel):
     """Thread model to group messages."""
 
-    subject = models.CharField(_("subject"), max_length=255)
+    subject = models.CharField(_("subject"), max_length=255, null=True, blank=True)
     snippet = models.TextField(_("snippet"), blank=True)
     has_unread = models.BooleanField(_("has unread"), default=False)
     has_trashed = models.BooleanField(_("has trashed"), default=False)
@@ -344,7 +344,7 @@ class Meta:
         verbose_name_plural = _("threads")
 
     def __str__(self):
-        return self.subject
+        return str(self.subject) if self.subject else "(no subject)"
 
     def update_stats(self):
         """Update the denormalized stats of the thread."""
@@ -693,7 +693,7 @@ class Message(BaseModel):
     thread = models.ForeignKey(
         Thread, on_delete=models.CASCADE, related_name="messages"
     )
-    subject = models.CharField(_("subject"), max_length=255)
+    subject = models.CharField(_("subject"), max_length=255, null=True, blank=True)
     sender = models.ForeignKey("Contact", on_delete=models.CASCADE)
     parent = models.ForeignKey(
         "Message", on_delete=models.SET_NULL, null=True, blank=True
@@ -733,7 +733,7 @@ class Meta:
         ordering = ["-created_at"]
 
     def __str__(self):
-        return self.subject
+        return str(self.subject) if self.subject else "(no subject)"
 
     def get_parsed_data(self) -> Dict[str, Any]:
         """Parse raw_mime using parser and cache the result."""

src/backend/core/tests/api/test_messages_create.py

@@ -476,16 +476,170 @@ def test_draft_message_unauthorized(self):
                 "senderId": uuid.uuid4(),
                 "subject": "test",
                 "draftBody": "<p>test</p> or test",
-                "to": ["pierre@example.com"],
+                "to": ["pierre@external.com"],
             },
             format="json",
         )
         # Assert the response is unauthorized
         assert response.status_code == status.HTTP_401_UNAUTHORIZED
 
-    # TODO: implement this test
-    # def test_send_message_unauthorized(self, mailbox, authenticated_user, send_url):
-    #    """Test send message unauthorized."""
+    def test_draft_message_with_empty_subject(self, mailbox, authenticated_user):
+        """Test create draft message with empty subject."""
+        factories.MailboxAccessFactory(
+            mailbox=mailbox,
+            user=authenticated_user,
+            role=enums.MailboxRoleChoices.EDITOR,
+        )
+
+        client = APIClient()
+        client.force_authenticate(user=authenticated_user)
+
+        draft_response = client.post(
+            reverse("draft-message"),
+            {
+                "senderId": mailbox.id,
+                "subject": "",  # Empty subject should be allowed
+                "draftBody": "Test content",
+                "to": ["[email protected]"],
+            },
+            format="json",
+        )
+
+        assert draft_response.status_code == status.HTTP_201_CREATED
+        draft_message_id = draft_response.data["id"]
+
+        # Verify the message was created with empty subject
+        draft_message = models.Message.objects.get(id=draft_message_id)
+        assert draft_message.subject == ""
+        assert draft_message.is_draft is True
+        assert str(draft_message) == "(no subject)"
+
+        # Verify the thread also has empty subject
+        assert draft_message.thread.subject == ""
+        assert str(draft_message.thread) == "(no subject)"
+
+    def test_draft_message_without_subject(self, mailbox, authenticated_user):
+        """Test create draft message without subject field."""
+        factories.MailboxAccessFactory(
+            mailbox=mailbox,
+            user=authenticated_user,
+            role=enums.MailboxRoleChoices.EDITOR,
+        )
+
+        client = APIClient()
+        client.force_authenticate(user=authenticated_user)
+
+        draft_response = client.post(
+            reverse("draft-message"),
+            {
+                "senderId": mailbox.id,
+                # No subject field - should default to None
+                "draftBody": "Test content",
+                "to": ["[email protected]"],
+            },
+            format="json",
+        )
+
+        assert draft_response.status_code == status.HTTP_201_CREATED
+        draft_message_id = draft_response.data["id"]
+
+        # Verify the message was created with None subject
+        draft_message = models.Message.objects.get(id=draft_message_id)
+        assert draft_message.subject is None
+        assert draft_message.is_draft is True
+        assert str(draft_message) == "(no subject)"
+
+        # Verify the thread also has None subject
+        assert draft_message.thread.subject is None
+        assert str(draft_message.thread) == "(no subject)"
+
+    @patch("core.mda.outbound.send_outbound_message")
+    def test_send_message_with_empty_subject(
+        self, mock_send_outbound_message, mailbox, authenticated_user, send_url
+    ):
+        """Test sending a message with empty subject (migration 0018)."""
+        mock_send_outbound_message.side_effect = lambda recipient_emails, message: {
+            recipient_email: {
+                "delivered": True,
+                "error": None,
+            }
+            for recipient_email in recipient_emails
+        }
+
+        factories.MailboxAccessFactory(
+            mailbox=mailbox,
+            user=authenticated_user,
+            role=enums.MailboxRoleChoices.EDITOR,
+        )
+
+        client = APIClient()
+        client.force_authenticate(user=authenticated_user)
+
+        # Create draft with empty subject
+        draft_response = client.post(
+            reverse("draft-message"),
+            {
+                "senderId": mailbox.id,
+                "subject": "",  # Empty subject
+                "draftBody": "Test content",
+                "to": ["[email protected]"],
+            },
+            format="json",
+        )
+
+        assert draft_response.status_code == status.HTTP_201_CREATED
+        draft_message_id = draft_response.data["id"]
+
+        # Send the message
+        send_response = client.post(
+            send_url,
+            {
+                "messageId": draft_message_id,
+                "senderId": mailbox.id,
+            },
+            format="json",
+        )
+
+        assert send_response.status_code == status.HTTP_200_OK
+
+        # Verify the message was sent with empty subject
+        sent_message = models.Message.objects.get(id=draft_message_id)
+        assert sent_message.subject == ""
+        assert sent_message.is_draft is False
+        assert sent_message.sent_at is not None
+        assert str(sent_message) == "(no subject)"
+
+        # Verify the thread also has empty subject
+        assert sent_message.thread.subject == ""
+        assert str(sent_message.thread) == "(no subject)"
+
+    def test_draft_message_with_very_long_subject(self, mailbox, authenticated_user):
+        """Test create draft message with subject exceeding max_length."""
+        factories.MailboxAccessFactory(
+            mailbox=mailbox,
+            user=authenticated_user,
+            role=enums.MailboxRoleChoices.EDITOR,
+        )
+
+        client = APIClient()
+        client.force_authenticate(user=authenticated_user)
+
+        # Create a subject that exceeds max_length (255)
+        long_subject = "A" * 256
+
+        draft_response = client.post(
+            reverse("draft-message"),
+            {
+                "senderId": mailbox.id,
+                "subject": long_subject,
+                "draftBody": "Test content",
+                "to": ["[email protected]"],
+            },
+            format="json",
+        )
+
+        # Should fail due to max_length constraint
+        assert draft_response.status_code == status.HTTP_400_BAD_REQUEST
 
     def test_send_nonexistent_message(self, mailbox, authenticated_user, send_url):
         """Test sending a message that does not exist."""