I have a module that has a model that inherits the sales.order model and adds some fields into it. Now I want to be able to show the value of one of those fields in each activity that is created within each quotation/sales order. 

I have tried using a related field in an inherited mail.activity model, but it seems to cause an internal error.

My thought as to why it causes an internal error is because the field I created only exists in the sales.order model and not in the other models that have activities.

Any help would be much appreciated.