Decorating the ActiveRecord @config object in a thread-safe manner

Not sure if this is needed as @config[:database] doesn't seem to affect any tests.

lib/active_record_host_pool/connection_adapter_mixin.rb

@@ -3,17 +3,55 @@
 require "active_record/connection_adapters/mysql2_adapter"
 
 module ActiveRecordHostPool
+  module PerThreadHashAccess
+    # PerThreadHashAccess.module_for(...) returns a module which scopes hash accessor methods
+    # to the given connection_id and uses `super` as a fallbakc. This is to make the access
+    # to the config object on a ActiveRecord::ConnectionAdapters::Mysql2Adapter thread-safe.
+    #
+    # If any of the config key/value(s) change then those changes will be used for that thread.
+    # It will fallback to config's original key/value pairs if none have been set in a thread specific way.
+    #
+    # Note: A module is being used in order to remain ignorant about how/where the config object is used in
+    # ActiveRecord itself.
+    def self.module_for(connection_id)
+      generate_storage_key = ->(key) { "config/#{key}" }
+
+      Module.new do
+        define_method(:[]=) do |key, value|
+          storage_key = generate_storage_key.call(key)
+          DatabaseSwitch.thread_local_storage[connection_id][storage_key] = value
+        end
+
+        define_method(:[]) do |key|
+          storage_key = generate_storage_key.call(key)
+          if DatabaseSwitch.thread_local_storage[connection_id].key?(storage_key)
+            DatabaseSwitch.thread_local_storage[connection_id][storage_key]
+          else
+            DatabaseSwitch.thread_local_storage[connection_id][storage_key] = super(key)
+          end
+        end
+
+        define_method(:fetch) do |*args, &blk|
+          key, _ = args
+          storage_key = generate_storage_key.call(key)
+          DatabaseSwitch.thread_local_storage[connection_id].fetch(storage_key) do
+            DatabaseSwitch.thread_local_storage[connection_id][storage_key] = super(*args, &blk)
+          end
+        end
+      end
+    end
+  end
+
   module DatabaseSwitch
     def self.included(base)
       base.class_eval do
-        attr_reader :_host_pool_current_database
         def _host_pool_current_database
           _ahrp_per_thread_per_connection_storage[:_host_pool_current_database]
         end
 
         def _host_pool_current_database=(database)
           _ahrp_per_thread_per_connection_storage[:_host_pool_current_database] = database
-          # @config[:database] = _host_pool_current_database if ActiveRecord::VERSION::MAJOR >= 5
+          @config[:database] = _host_pool_current_database if ActiveRecord::VERSION::MAJOR >= 5
         end
 
         alias_method :execute_without_switching, :execute
@@ -37,6 +75,7 @@ def self.thread_local_storage
     def initialize(*)
       self._cached_current_database = nil
       super
+      @config.singleton_class.prepend PerThreadHashAccess.module_for(@connection.object_id)
     end
 
     def execute_with_switching(*args)