Rails: activeadmin overriding create action

I ran into a similar situation where I didn’t really need to completely override the create method. I really only wanted to inject properties before save, and only on create; very similar to your example. After reading through the ActiveAdmin source, I determined that I could use before_create to do what I needed:

ActiveAdmin.register Product do
  before_create do |product|
    product.creator = current_user
  end
end