use arel nodes to represent non-string `order_values`.

This fixes a bug when merging relations of different classes.

```
Given:
  Post.joins(:author).merge(Author.order(name: :desc)).to_sql

Before:
 SELECT "posts".* FROM "posts"
   INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
   ORDER BY "posts"."name" DESC

After:
 SELECT "posts".* FROM "posts"
   INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
   ORDER BY "authors"."name" DESC
```

2 files changed, 13 insertions, 5 deletions

diff --git a/activerecord/test/cases/relation/merging_test.rb b/activerecord/test/cases/relation/merging_test.rb
index 9f9244f9b2..23500bf5d8 100644
--- a/activerecord/test/cases/relation/merging_test.rb
+++ b/activerecord/test/cases/relation/merging_test.rb
@@ -146,5 +146,17 @@ class MergingDifferentRelationsTest < ActiveRecord::TestCase
       merge(Author.order(:name)).pluck("authors.name")
 
     assert_equal ["Bob", "Bob", "David"], posts_by_author_name
+
+    posts_by_author_name = Post.limit(3).joins(:author).
+      merge(Author.order("name")).pluck("authors.name")
+
+    assert_equal ["Bob", "Bob", "David"], posts_by_author_name
+  end
+
+  test "merging order relations (using a hash argument)" do
+    posts_by_author_name = Post.limit(4).joins(:author).
+      merge(Author.order(name: :desc)).pluck("authors.name")
+
+    assert_equal ["Mary", "Mary", "Mary", "David"], posts_by_author_name
   end
 end
diff --git a/activerecord/test/cases/relation/mutation_test.rb b/activerecord/test/cases/relation/mutation_test.rb
index 020fb24afa..a70f979442 100644
--- a/activerecord/test/cases/relation/mutation_test.rb
+++ b/activerecord/test/cases/relation/mutation_test.rb
@@ -7,10 +7,6 @@ module ActiveRecord
       extend ActiveRecord::Delegation::DelegateCache
       inherited self
 
-      def arel_table
-        Post.arel_table
-      end
-
       def connection
         Post.connection
       end
@@ -21,7 +17,7 @@ module ActiveRecord
     end
 
     def relation
-      @relation ||= Relation.new FakeKlass.new('posts'), :b
+      @relation ||= Relation.new FakeKlass.new('posts'), Post.arel_table
     end
 
     (Relation::MULTI_VALUE_METHODS - [:references, :extending, :order]).each do |method|