Thanks Cyril ! This is excellent.

Just a couple of changes that I had to make from my end:

1. The function name beginning with underscore (_get_partner_ids) had to be changed to just get_partner_ids because the private class was inaccessible from the template.

2. partner.id should be partner_id.id

3. Calling the function from template was defined without the cr,uid because that was raising issues for me

   ${object.get_partner_ids(object.user_ids)}

4. Also, could you tell me why the additional char field is required?
   

Thanks again !

I have a similar case, but can't figure out how I get the correct partner ids. I have a message model that is related to project.project and I want to get the followers of the project into my mail.template.

Here is my code so far. I stuck at the relation between the field project.message_follower_ids and  res_partner.  (project.message_follower_ids is related to mail.followers -> res_id).

<field name="partner_to">${object.get_partner_ids(object.project_id.message_follower_ids)}</field>

@api.multi  # TODO get the followers of the active Scrum Project
def get_partner_ids(self, user_ids):
    return str([user_ids.ids]).replace('[', '').replace(']', '')