Define arel_visitor method on all adapters

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -149,7 +149,7 @@ def collector
       end
 
       def arel_visitor # :nodoc:
-        (Arel::Visitors::VISITORS[@config[:adapter]] || Arel::Visitors::ToSql).new(self)
+        Arel::Visitors::ToSql.new(self)
       end
 
       def valid_type?(type)

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -24,6 +24,10 @@ def schema_creation
         MySQL::SchemaCreation.new(self)
       end
 
+      def arel_visitor # :nodoc:
+        Arel::Visitors::MySQL.new(self)
+      end
+
       ##
       # :singleton-method:
       # By default, the Mysql2Adapter will consider all columns of type <tt>tinyint(1)</tt>

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -125,6 +125,10 @@ def schema_creation # :nodoc:
         PostgreSQL::SchemaCreation.new self
       end
 
+      def arel_visitor # :nodoc:
+        Arel::Visitors::PostgreSQL.new(self)
+      end
+
       # Returns true, since this connection adapter supports prepared statement
       # caching.
       def supports_statement_cache?

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -80,6 +80,10 @@ def schema_creation # :nodoc:
         SQLite3::SchemaCreation.new self
       end
 
+      def arel_visitor # :nodoc:
+        Arel::Visitors::SQLite.new(self)
+      end
+
       def initialize(connection, logger, connection_options, config)
         super(connection, logger, config)