I'm creating a helper to be used by Formtastic but I get the undefined local variable or method
error. I don't know where to put it so it can work.
I already tried in the application_helper.rb and in app/helpers/active_admin/view_helpers.rb
You can define them in app/helpers/ as you tried but you need to include them trough the active admin's initializer like this:
# in config/initializers/active_admin.rb
ActiveAdmin.setup do |config|
....
end
module ActiveAdmin::ViewHelpers
include ApplicationHelper
end