Replace global Hash usage with TS::Cache.

Rakefile

@@ -17,4 +17,6 @@ Hoe.spec 'arel' do
   self.licenses         = ['MIT']
   self.readme_file      = 'README.markdown'
   self.extra_rdoc_files = FileList['README.markdown']
+
+  dependency('thread_safe', '~> 0.1')
 end

lib/arel/visitors/to_sql.rb

@@ -1,5 +1,6 @@
 require 'bigdecimal'
 require 'date'
+require 'thread_safe'
 
 module Arel
   module Visitors
@@ -56,8 +57,8 @@ class ToSql < Arel::Visitors::Visitor
       def initialize connection
         @connection     = connection
         @schema_cache   = connection.schema_cache
-        @quoted_tables  = {}
-        @quoted_columns = {}
+        @quoted_tables  = ThreadSafe::Cache.new
+        @quoted_columns = ThreadSafe::Cache.new
       end
 
       private

lib/arel/visitors/visitor.rb

@@ -1,3 +1,5 @@
+require 'thread_safe'
+
 module Arel
   module Visitors
     class Visitor
@@ -7,10 +9,10 @@ def accept object
 
       private
 
-      DISPATCH = Hash.new do |hash, visitor_class|
+      DISPATCH = ThreadSafe::Cache.new do |hash, visitor_class|
         hash[visitor_class] =
-          Hash.new do |hash, node_class|
-            hash[node_class] = "visit_#{(node_class.name || '').gsub('::', '_')}"
+          ThreadSafe::Cache.new(:initial_capacity => 2) do |inner_hash, node_class|
+            inner_hash[node_class] = "visit_#{(node_class.name || '').gsub('::', '_')}"
           end
       end