Doesn't work on Windows 7, but on Ubuntu 12.04 and ArchLinux. OpenJDK 7 and Oracle JDK 7. Latest JRuby and Gems.

gistfile1.rb

# encoding: utf-8

require "active_record"
#require "activerecord-jdbcsqlite3-adapter" # gem install this, but not require
require 'activerecord-jdbc-adapter'

# a very basic migration
class CreateSamplesTable < ActiveRecord::Migration
  def change
    create_table :samples do |t|
      t.integer :num
    end
  end
end

# minimal class
class Sample < ActiveRecord::Base
end

# connect to the database
ActiveRecord::Base.establish_connection(
  :adapter => 'jdbcsqlite3',
  #:adapter => 'sqlite3',
  #:database => ":memory:",
  :database => ":memory:?cache=shared",
  #:url => "jdbc:sqlite:file::memory:;cache=shared",
  #:cache => "shared",
  #:database => "db_file",
  #url: "jdbc:sqlite::memory:",
  :wait_timeout => 1,
  :timeout => 250,
  #:reconnect =>  true,
  :encoding => "utf-8",
  :pool => 10
)

# migrate 
CreateSamplesTable.migrate :up unless Sample.table_exists?

# populate the table
Sample.create num: 17 if Sample.all.empty?

Thread.new do
  # output the same. 
	# without the shared cache this will throw an error
	# because every thread rebuilds the database
	begin
		puts "number of samples: #{Sample.count}"
	rescue => e 
		# capture the exception, since raises in a different thread won't show up
		puts "ERROR from Thread: #{e.message}"
	end
end

# output. should be 1 since its the main thread
puts "number of samples: #{Sample.count}"

sleep 5