quoting update values using the column

lib/arel/nodes/unqualified_column.rb

@@ -5,6 +5,10 @@ def initialize attribute
         @attribute = attribute
       end
 
+      def column
+        @attribute.column
+      end
+
       def name
         @attribute.name
       end

lib/arel/visitors/to_sql.rb

@@ -179,9 +179,7 @@ def visit_Arel_Nodes_Or o
       end
 
       def visit_Arel_Nodes_Assignment o
-        right = o.right
-
-        right = right.nil? ? 'NULL' : visit(right)
+        right = quote(o.right, o.left.column)
         "#{visit o.left} = #{right}"
       end