How to set model validation message to not include attribute name

Question

I'm trying to show the user the error message "Arrangement has already been scheduled for this case" when a case_id is not unique on an Arrangement.

Here's my en.yml file:

en:
  activerecord:
    errors:
      models:
        arrangement:
          attributes:
            case_id: 
              taken: "Arrangement has already been scheduled for this case"

But i keep getting the message "Case Arrangement has already been scheduled for this case".

How can i NOT show the field name in this error message?

Arrangement model

class Arrangement < ActiveRecord::Base
  belongs_to :case
  validates_uniqueness_of :case_id, :allow_nil => true
...

EDIT: The links suggested don't seem to work on a per model basis.

i.e. this does not seem to work. It still shows the message "Case Arrangement has already been scheduled for this case".

en:
  activerecord:
    errors:
      models:
        arrangement:
          format: "%{message}"
          attributes:
            case_id: 
              taken: "Arrangement has already been scheduled for this case"