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
```

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use strings to represent non-string `order_values`.
+
+    *Yves Senn*
+
 *   Checks to see if the record contains the foreign key to set the inverse automatically.
 
     *Edo Balvers*

activerecord/lib/active_record/relation/query_methods.rb

@@ -995,12 +995,6 @@ module ActiveRecord
             s.strip!
             s.gsub!(/\sasc\Z/i, ' DESC') || s.gsub!(/\sdesc\Z/i, ' ASC') || s.concat(' DESC')
           end
-        when Symbol
-          { o => :desc }
-        when Hash
-          o.each_with_object({}) do |(field, dir), memo|
-            memo[field] = (dir == :asc ? :desc : :asc )
-          end
         else
           o
         end
@@ -1016,17 +1010,6 @@ module ActiveRecord
       orders.reject!(&:blank?)
       orders = reverse_sql_order(orders) if reverse_order_value
 
-      orders = orders.flat_map do |order|
-        case order
-        when Symbol
-          table[order].asc
-        when Hash
-          order.map { |field, dir| table[field].send(dir) }
-        else
-          order
-        end
-      end
-
       arel.order(*orders) unless orders.empty?
     end
 
@@ -1048,8 +1031,17 @@ module ActiveRecord
 
       # if a symbol is given we prepend the quoted table name
       order_args.map! do |arg|
-        arg.is_a?(Symbol) ? Arel::Nodes::Ascending.new(klass.arel_table[arg]) : arg
+        case arg
+        when Symbol
+          table[arg].asc
+        when Hash
+          arg.map { |field, dir|
+            table[field].send(dir)
+          }
+        else
+          arg
         end
+      end.flatten!
     end
 
     # Checks to make sure that the arguments are not blank. Note that if some

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

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|