If you want to set a context for a field of another model via a O2M, you have the possibility to set the context directly on this O2M and not in the related model. 

An example would be to auto-fill the partner_id field of the timesheets (timesheet_ids) on a ticket. You can open Studio, select the timesheet_ids field and directly add the default value for the partner_id in the context of this field: