First, as you already know, make sure you use the correct ruby version
rvm list
Then (if, for example, you used p249)
rvm ruby-1.8.7-p249
If this does not work, it's time to be a little adventurous and debug the core of the rails. The put method helps a lot here.
, , ( , mix-ins, ).
:
rake gems:unpack
rake gems:install