Add attachment_data_id to file attachments

This needs to follow on from alphagov/publishing-api#2994

As described there, the lack of attachment_data_id in the data for file asset attachments in content store makes it hard to request the attachments on the server side. Instead, we have to redirect users to the assets, which has led to ugly workarounds like CSV previews being rendered by frontend but served on assets.publishing.service.gov.uk.

Adding attachment_data_id should allow the frontends to request attachments directly, using the API client:

    GdsApi.asset_manager.media(attachment_data_id, filename)

This should make it much more convenient to preview assets, or use them for other rendering purposes (e.g. showing a CSV as a line graph).

app/models/file_attachment.rb

@@ -55,6 +55,7 @@ def publishing_api_details_for_format
       filename:,
       number_of_pages:,
       preview_url:,
+      attachment_data_id:,
     }
   end
 
@@ -66,6 +67,12 @@ def alternative_format_contact_email
     nil
   end
 
+  def attachment_data_id
+    return unless csv? && attachable.is_a?(Edition) && attachment_data.all_asset_variants_uploaded?
+
+    attachment_data.id
+  end
+
   def preview_url
     return unless csv? && attachable.is_a?(Edition) && attachment_data.all_asset_variants_uploaded?
 

test/unit/app/models/file_attachment_test.rb

@@ -68,4 +68,9 @@ def assert_delegated(attachment, method)
     attachment = create(:csv_attachment, attachable: create(:edition))
     assert_equal Plek.asset_root + "/media/#{attachment.attachment_data.id}/sample.csv/preview", attachment.publishing_api_details_for_format[:preview_url]
   end
+
+  test "return media attachment_data_id if all_asset_variants_uploaded?" do
+    attachment = create(:csv_attachment, attachable: create(:edition))
+    assert_equal attachment.attachment_data.id, attachment.publishing_api_details_for_format[:attachment_data_id]
+  end
 end