ssh to execute remote ruby script

I'm using 'net/ssh' to ssh into a OS X server and run a ruby script,
like this:

  Net::SSH.start( "xxx.xxx.xxx", :username => 'xxx', :password => 'xxx'
) do | session |
    shell = session.shell.open
    shell.exec "ruby /usr/local/pgsql/share/migrate.rb
#{params['host']}"
  end

The migrate.rb looks like this:

  require 'rubygems'
  ...some ruby code

For some reason, the script fails to execute anything after the require
'rubygems' line.  If I remove that line, the script will run, but
obviously I cannot use any gems, which I need.

If I ssh via terminal to the server, I can run the script fine with:
ruby /usr/local/pgsql/share/migrate.rb 1

I can't figure out why, when ssh'ing via the net/ssh package, the remote
script cannot include rubygems.

Any ideas?
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