From 66b19b5dec78a74e4280d69ac0e4801699ca691a Mon Sep 17 00:00:00 2001
From: Kevin Deisz <kevin.deisz@gmail.com>
Date: Tue, 31 Dec 2019 15:59:59 -0500
Subject: [PATCH] Allow #nulls_first and #nulls_last in PostgreSQL

When using PostgreSQL, it's useful to be able to specify NULLS FIRST and NULLS LAST on ordered columns. With this commit you can do that now, as in:

```ruby
User.arel_table[:first_name].desc.nulls_last
```
---
 activerecord/lib/arel/nodes.rb                 |  1 +
 activerecord/lib/arel/nodes/ordering.rb        | 18 ++++++++++++++++++
 activerecord/lib/arel/nodes/unary.rb           |  1 -
 activerecord/lib/arel/visitors/postgresql.rb   | 10 ++++++++++
 .../test/cases/arel/visitors/postgres_test.rb  | 16 ++++++++++++++++
 5 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 activerecord/lib/arel/nodes/ordering.rb

diff --git a/activerecord/lib/arel/nodes.rb b/activerecord/lib/arel/nodes.rb
index f994754620..e7c1acc30a 100644
--- a/activerecord/lib/arel/nodes.rb
+++ b/activerecord/lib/arel/nodes.rb
@@ -18,6 +18,7 @@ require "arel/nodes/false"
 # unary
 require "arel/nodes/unary"
 require "arel/nodes/grouping"
+require "arel/nodes/ordering"
 require "arel/nodes/ascending"
 require "arel/nodes/descending"
 require "arel/nodes/unqualified_column"
diff --git a/activerecord/lib/arel/nodes/ordering.rb b/activerecord/lib/arel/nodes/ordering.rb
new file mode 100644
index 0000000000..4c4cb58c9f
--- /dev/null
+++ b/activerecord/lib/arel/nodes/ordering.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Nodes
+    class Ordering < Unary
+      def nulls_first
+        NullsFirst.new(self)
+      end
+
+      def nulls_last
+        NullsLast.new(self)
+      end
+    end
+
+    class NullsFirst < Ordering; end
+    class NullsLast < Ordering; end
+  end
+end
diff --git a/activerecord/lib/arel/nodes/unary.rb b/activerecord/lib/arel/nodes/unary.rb
index 6d1ac36b0e..cf1eca2831 100644
--- a/activerecord/lib/arel/nodes/unary.rb
+++ b/activerecord/lib/arel/nodes/unary.rb
@@ -36,7 +36,6 @@ module Arel # :nodoc: all
       Offset
       On
       OptimizerHints
-      Ordering
       RollUp
     }.each do |name|
       const_set(name, Class.new(Unary))
diff --git a/activerecord/lib/arel/visitors/postgresql.rb b/activerecord/lib/arel/visitors/postgresql.rb
index d4f21ff93e..65eceeef74 100644
--- a/activerecord/lib/arel/visitors/postgresql.rb
+++ b/activerecord/lib/arel/visitors/postgresql.rb
@@ -82,6 +82,16 @@ module Arel # :nodoc: all
           visit o.right, collector
         end
 
+        def visit_Arel_Nodes_NullsFirst(o, collector)
+          visit o.expr, collector
+          collector << " NULLS FIRST"
+        end
+
+        def visit_Arel_Nodes_NullsLast(o, collector)
+          visit o.expr, collector
+          collector << " NULLS LAST"
+        end
+
         # Used by Lateral visitor to enclose select queries in parentheses
         def grouping_parentheses(o, collector)
           if o.expr.is_a? Nodes::SelectStatement
diff --git a/activerecord/test/cases/arel/visitors/postgres_test.rb b/activerecord/test/cases/arel/visitors/postgres_test.rb
index 34709ae7ea..e0096f52cb 100644
--- a/activerecord/test/cases/arel/visitors/postgres_test.rb
+++ b/activerecord/test/cases/arel/visitors/postgres_test.rb
@@ -315,6 +315,22 @@ module Arel
           _(sql).must_be_like %{ "users"."name" IS DISTINCT FROM NULL }
         end
       end
+
+      describe "Nodes::Ordering" do
+        it "should handle nulls first" do
+          test = Table.new(:users)[:first_name].desc.nulls_first
+          _(compile(test)).must_be_like %{
+            "users"."first_name" DESC NULLS FIRST
+          }
+        end
+
+        it "should handle nulls last" do
+          test = Table.new(:users)[:first_name].desc.nulls_last
+          _(compile(test)).must_be_like %{
+            "users"."first_name" DESC NULLS LAST
+          }
+        end
+      end
     end
   end
 end