Hi Everybody

We've ran into the following issue:

Invoice App:

We added a many2many field (additional_recipients) in our invoice app (account.move). It is filled from a similar field in res.partner, which contains the additional contacts for each partner.

The goal is, that invoices are automatically sent to the contacts, selected in this field.

Mail Template:

The field partner_to in the mail.template, now has the following value: 

{{ object.partner_id.id }}, {{ object.additional_recipients.id }}

Error Messages:

In case the there are no additional contacts, the mail template runs into a value error because there is no ID from which it would get an email-address

ValueError: invalid literal for int() with base 10: ' ' 

In case there are multiple contacts selected, it runs into an error because the partner_to field expects singleton expressions. Meaning, that it works as long as there is only one contact in the field. 

Failed to render inline_template template : : "Expected singleton: res.partner(2728, 3374)" while evaluating' object.additional_recipients.id ')

Do you have an idea how to fix this issue without coding too much?

Kind regards, and thank you in advance!