use thread locals and an instance variable within QueryCache#BodyProxy to maintain appropriate linkage with AR database connection across threads

Author: mjtko

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -314,7 +314,7 @@ def new_connection
       end
 
       def current_connection_id #:nodoc:
-        Thread.current.object_id
+        ActiveRecord::Base.connection_id ||= Thread.current.object_id
       end
 
       def checkout_new_connection

activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb

@@ -115,6 +115,14 @@ def connection
         retrieve_connection
       end
 
+      def connection_id
+        Thread.current['ActiveRecord::Base.connection_id']
+      end
+
+      def connection_id=(connection_id)
+        Thread.current['ActiveRecord::Base.connection_id'] = connection_id
+      end
+
       # Returns the configuration of the associated connection as a hash:
       #
       #  ActiveRecord::Base.connection_config

activerecord/lib/active_record/query_cache.rb

@@ -28,9 +28,10 @@ def initialize(app)
     end
 
     class BodyProxy # :nodoc:
-      def initialize(original_cache_value, target)
+      def initialize(original_cache_value, target, connection_id)
         @original_cache_value = original_cache_value
         @target               = target
+        @connection_id        = connection_id
       end
 
       def method_missing(method_sym, *arguments, &block)
@@ -48,6 +49,7 @@ def each(&block)
       def close
         @target.close if @target.respond_to?(:close)
       ensure
+        ActiveRecord::Base.connection_id = @connection_id
         ActiveRecord::Base.connection.clear_query_cache
         unless @original_cache_value
           ActiveRecord::Base.connection.disable_query_cache!
@@ -60,7 +62,7 @@ def call(env)
       ActiveRecord::Base.connection.enable_query_cache!
 
       status, headers, body = @app.call(env)
-      [status, headers, BodyProxy.new(old, body)]
+      [status, headers, BodyProxy.new(old, body, ActiveRecord::Base.connection_id)]
     rescue Exception => e
       ActiveRecord::Base.connection.clear_query_cache
       unless old