I have a project working on Ruby-1.9.3-p392 and i try to load it with Jruby (jruby-head installed with rvm, it is currently jruby 1.7.4.dev (1.9.3p392))
All rake tasks fail with the following trace
bundle exec rake -T rake aborted! load error: pg_ext -- java.lang.RuntimeException: C extension initialized against invalid ruby runtime org/jruby/RubyKernel.java:1028:in `require' /home/dmitry/.rvm/gems/jruby-head/gems/pg-0.14.1/lib/pg.rb:4:in `(root)' org/jruby/RubyKernel.java:1028:in `require' /home/dmitry/.rvm/gems/jruby-head/gems/bundler-1.3.4/lib/bundler/runtime.rb:1:in `(root)' org/jruby/RubyArray.java:1614:in `each' /home/dmitry/.rvm/gems/jruby-head/gems/bundler-1.3.4/lib/bundler/runtime.rb:72:in `require' org/jruby/RubyArray.java:1614:in `each' /home/dmitry/.rvm/gems/jruby-head/gems/bundler-1.3.4/lib/bundler/runtime.rb:70:in `require' /home/dmitry/.rvm/gems/jruby-head/gems/bundler-1.3.4/lib/bundler/runtime.rb:59:in `require' /home/dmitry/.rvm/gems/jruby-head/gems/bundler-1.3.4/lib/bundler.rb:132:in `require' org/jruby/RubyKernel.java:1028:in `require' /home/dmitry/Work/crossArts/config/application.rb:13:in `(root)' org/jruby/RubyKernel.java:1047:in `load' /home/dmitry/Work/crossArts/Rakefile:1:in `(root)' /home/dmitry/Work/crossArts/Rakefile:5:in `(root)' org/jruby/RubyKernel.java:1047:in `load' org/jruby/RubyKernel.java:1067:in `eval' /home/dmitry/.rvm/gems/jruby-head/bin/ruby_noexec_wrapper:14:in `(root)'
Try adding gem 'activerecord-jdbcpostgresql-adapter' to your Gemfile. That's what I did for a rails app on puma 2.0.1 and JRuby 1.7.4, and it worked like a champ.