raising not implemented exceptions for distinct on where it is not su…

…pported

lib/arel/visitors/to_sql.rb

@@ -142,6 +142,10 @@ def visit_Arel_Nodes_Distinct o
         'DISTINCT'
       end
 
+      def visit_Arel_Nodes_DistinctOn o
+        raise NotImplementedError, 'DISTINCT ON not implemented for this db'
+      end
+
       def visit_Arel_Nodes_With o
         "WITH #{o.children.map { |x| visit x }.join(', ')}"
       end

test/visitors/test_to_sql.rb

@@ -299,6 +299,17 @@ def quote value, column = nil
           }
         end
       end
+
+      describe 'distinct on' do
+        it 'raises not implemented error' do
+          core = Arel::Nodes::SelectCore.new
+          core.set_quantifier = Arel::Nodes::DistinctOn.new(Arel.sql('aaron'))
+
+          assert_raises(NotImplementedError) do
+            @visitor.accept(core)
+          end
+        end
+      end
     end
   end
 end