adding visitors to the respective adapters

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -127,6 +127,7 @@ module ActiveRecord
         super(connection, logger)
         @connection_options, @config = connection_options, config
         @quoted_column_names, @quoted_table_names = {}, {}
+        @visitor = Arel::Visitors::MySQL.new self
       end
 
       def self.visitor_for(pool) # :nodoc:

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -307,6 +307,7 @@ module ActiveRecord
       def initialize(connection, logger, connection_parameters, config)
         super(connection, logger)
         @connection_parameters, @config = connection_parameters, config
+        @visitor = Arel::Visitors::PostgreSQL.new self
 
         # @local_tz is initialized as nil to avoid warnings when connect tries to use it
         @local_tz = nil