[FIX] l10n_it_sdi_channel: Subscribe to related channel

l10n_it_sdi_channel/models/fatturapa_attachment.py

@@ -8,5 +8,31 @@ class FatturaPAAttachment(models.AbstractModel):
     _inherit = "fatturapa.attachment"
 
     channel_id = fields.Many2one(
-        comodel_name="sdi.channel", related="company_id.sdi_channel_id", store=True
+        comodel_name="sdi.channel",
+        related="company_id.sdi_channel_id",
+        store=True,
     )
+
+    def _message_auto_subscribe_followers(self, updated_values, default_subtype_ids):
+        res = super()._message_auto_subscribe_followers(
+            updated_values, default_subtype_ids
+        )
+        updated_channel_id = updated_values.get("channel_id")
+        if not updated_channel_id:
+            # SdI channel is not explicitly set, so SdI channel's followers
+            # won't be subscribed to current attachments.
+            # Attachments might have the SdI channel set anyway
+            # because it is a related field.
+            # Add the SdI channel's followers
+            # but only if all the attachments share the same SdI channel
+            channel = self.mapped("channel_id")
+            if all(attachment.channel_id == channel for attachment in self):
+                for partner_follower in channel.message_partner_ids:
+                    res.append(
+                        (
+                            partner_follower.id,
+                            default_subtype_ids,
+                            False,
+                        )
+                    )
+        return res