Hi,

In Odoo 18, the Chatter widget is designed to only display activities (activity_ids) that belong directly to the current record. This makes it difficult to include activities from child contacts (child_ids) when viewing a parent contact. Overriding the existing activity_ids field does not work properly, because it is a core computed field on mail.thread and Odoo relies on it for filters, reminders, and activity domains. If you override it to merge child activities, the built-in logic for things like “My Activities” or overdue activities will no longer function correctly.

To achieve the desired behavior, one option is to create a custom field such as all_activity_ids on res.partner, which combines the partner’s own activities with those of its children. The challenge is that the Chatter component always points to activity_ids by default, so replacing it requires a small JavaScript (OWL) extension to override the Chatter view specifically for partners. This method is the cleanest if you want full integration, but it involves frontend customization.

Another approach is to hide the standard activity list in Chatter for contacts and add a custom widget or section that displays all_activity_ids. This way, you still see the merged activities in the form view, without breaking the default behavior of Chatter. It requires less invasive changes but does mean contacts will have a slightly different layout compared to other models.

As a simpler fallback, you can add a smart button or a new tab on the contact form that shows all activities, combining those from the parent and child contacts. This avoids touching the Chatter at all, is much easier to implement, and still gives end users visibility of everything in one place. In short, if you need deep integration inside Chatter you will need to extend it with JavaScript, but if visibility is enough then a smart button or separate tab is safer and quicker to implement.

Hope it helps