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我使用了spree（https://github.com/spree/spree_paypal_express）上的spree paypal express扩展，对rake db：migrate有如下错误。Spree - Spree_Paypal_Express - superclass必须是一个类（给出的模块）
的Ruby 1.8.7和3.1.3轨道
耙中止！ 超类必须是一个类（假设模块）
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:456:in `load_file'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:640:in `new_constants_in'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:455:in `load_file'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:342:in `require_or_load'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:489:in `load_missing_constant'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:181:in `const_missing'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:179:in `each'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/dependencies.rb:179:in `const_missing'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/bundler/gems/spree_paypal_express-6e6b553313c5/lib/spree_paypal_express/engine.rb:20
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/lazy_load_hooks.rb:34:in `call'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/lazy_load_hooks.rb:34:in `execute_hook'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/lazy_load_hooks.rb:43:in `run_load_hooks'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/lazy_load_hooks.rb:42:in `each'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/activesupport-3.1.1/lib/active_support/lazy_load_hooks.rb:42:in `run_load_hooks'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/application/finisher.rb:56
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/initializable.rb:30:in `instance_exec'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/initializable.rb:30:in `run'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/initializable.rb:55:in `run_initializers'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/initializable.rb:54:in `each'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/initializable.rb:54:in `run_initializers'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/application.rb:96:in `initialize!'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/railtie/configurable.rb:30:in `send'
/Users/kunalsupe/Developer/.rvm/gems/ruby-1.8.7-p302/gems/railties-3.1.1/lib/rails/railtie/configurable.rb:30:in `method_missing'
是否有这个