I've set the order (there was no order) of the sunspot_rails.init Initializer to run just before any application Initializers are loaded. This way Engines and Railties like rails_admin that make use of a Model's code will work correctly. See sferik/rails_admin#1018
The alternative to this is to run the contents of the init.rb file in a different Initilializer just before the one that uses any Model Class.
Change initializer order
Fix correct init hook order
Nice. I'll merge this into 1-3-stable and master.