Deleting Paperclip Attachments with ActiveAdmin

This technique will be consistent with the underlying mechanism used by Rails' built-in nested attributes for associations.

This example will use an Author model with an existing avatar attachment that we want to be able to delete in ActiveAdmin:

class Author < ApplicationRecord
  ...
  has_attached_file :avatar
  ...
end

Step 1: Add <attachment_name>_attributes= writer method

As accepts_nested_attributes_for only works with ActiveRecord associations like has_one, has_many, etc. we will not make direct use of it, but manually add the writer method it generates itself.

(To understand how nested attributes work in Rails see http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html).

Add a <attachment_name>_attributes= writer method like below to your model that already uses has_attached_file:

class Author < ApplicationRecord
  ...
  has_attached_file :avatar

+ def avatar_attributes=(attributes)
+   # Marks the attachment for destruction on next save,
+   # if the attributes hash contains a _destroy flag
+   # and a new avatar was not uploaded at the same time:
+   avatar.clear if has_destroy_flag?(attributes) && !avatar.dirty?
+ end
end

The method calls Paperclip::Attachment#clear to mark the attachment for destruction when the model is next saved.

Step 2: Setup ActiveAdmin strong params

Open app/admin/authors.rb (use the correct file path for your app) and setup strong parameters to permit the _destroy flag nested attribute:

# File: app/admin/authors.rb
ActiveAdmin.register Author do

-  permit_params :name, :bio, :avatar
+  permit_params :name, :bio, :avatar, avatar_attributes: [:_destroy]

Step 3: Add nested _destroy checkbox to ActiveAdmin form

In app/admin/authors.rb setup the form block to include a _destroy checkbox. This checkbox must be nested within <attachment_name>_attributes using semantic_fields_for (or one of the other nested attributes methods provided by formtastic).

  form do |f|
    f.semantic_errors
    f.inputs do
      f.input :name
      f.input :bio
      f.input :avatar, as: :file
+     if f.object.avatar.present?
+       f.semantic_fields_for :avatar_attributes do |avatar_fields|
+         avatar_fields.input :_destroy, as: :boolean, label: 'Delete?'
+       end
+     end
    end
    f.actions
  end

Your form should now show a delete checkbox when there is an attachment present. Checking this checkbox and submitting a valid form ought to delete the attachment.