Arel/MySQL ordering `nulls_first` and `nulls_last`

Fixes #50079 where there is unexpected behaviour for MySQL

Previously:
 - Calling nulls_first on asc worked as expected
 - Calling nulls_first on desc ordered desc nulls last
 - Calling nuls_last on asc raised error
 - Calling nulls_last on desc raised error

Now:
 - Calling nulls_first on asc works as expected
 - Calling nulls_first on desc works as expected
 - Calling nulls_last on asc works as expected
 - Calling nulls_last on desc works as expected

activerecord/CHANGELOG.md

@@ -84,4 +84,8 @@
 
     *Jason Meller*
 
+*   Add `nulls_last` and working `desc.nulls_first` for MySQL.
+
+    *Tristan Fellows*
+
 Please check [7-1-stable](https://github.com/rails/rails/blob/7-1-stable/activerecord/CHANGELOG.md) for previous changes.

activerecord/lib/arel/visitors/mysql.rb

@@ -59,9 +59,14 @@ module Arel # :nodoc: all
           infix_value o, collector, " NOT REGEXP "
         end
 
-        # no-op
         def visit_Arel_Nodes_NullsFirst(o, collector)
-          visit o.expr, collector
+          visit(o.expr.expr, collector) << " IS NOT NULL, "
+          visit(o.expr, collector)
+        end
+
+        def visit_Arel_Nodes_NullsLast(o, collector)
+          visit(o.expr.expr, collector) << " IS NULL, "
+          visit(o.expr, collector)
         end
 
         def visit_Arel_Nodes_Cte(o, collector)

activerecord/test/cases/arel/visitors/mysql_test.rb

@@ -154,10 +154,31 @@ module Arel
       end
 
       describe "Nodes::Ordering" do
-        it "should no-op ascending nulls first" do
+        it "should handle nulls first" do
           test = Table.new(:users)[:first_name].asc.nulls_first
           _(compile(test)).must_be_like %{
-            "users"."first_name" ASC
+            "users"."first_name" IS NOT NULL, "users"."first_name" ASC
+          }
+        end
+
+        it "should handle nulls last" do
+          test = Table.new(:users)[:first_name].asc.nulls_last
+          _(compile(test)).must_be_like %{
+            "users"."first_name" IS NULL, "users"."first_name" ASC
+          }
+        end
+
+        it "should handle nulls first reversed" do
+          test = Table.new(:users)[:first_name].asc.nulls_first.reverse
+          _(compile(test)).must_be_like %{
+            "users"."first_name" IS NULL, "users"."first_name" DESC
+          }
+        end
+
+        it "should handle nulls last reversed" do
+          test = Table.new(:users)[:first_name].asc.nulls_last.reverse
+          _(compile(test)).must_be_like %{
+            "users"."first_name" IS NOT NULL, "users"."first_name" DESC
           }
         end
       end