Rails Protips Volume 1 - asset_url
Using asset_url helper outside of views.
If you get the error
NoMethodError: undefined method `protocol' for nil:NilClass while using
ActionController::Base.helpers.asset_url(IMAGE_NAME) somewhere other than a view, change it to
ActionController::Base.helpers.asset_path(IMAGE_NAME). This is [probably] because your model doesn't have access to the request to figure out the protocol.