Unfortunately, it appears that the newest versions of ActiveRecord (3.2.0.rc1, at least) use:
ActiveRecord::ConnectionAdapters::Mysql2Adapter::Column
gems/spatial_adapter-1.2.0/lib/spatial_adapter/mysql2.rb:83:in <module:ConnectionAdapters>': uninitialized constant ActiveRecord::ConnectionAdapters::Mysql2Column (NameError) from gems/spatial_adapter-1.2.0/lib/spatial_adapter/mysql2.rb:82:inmodule:ActiveRecord'
from gems/spatial_adapter-1.2.0/lib/spatial_adapter/mysql2.rb:81:in <top (required)>' from gems/bundler-1.0.21/lib/bundler/runtime.rb:68:inrequire'