Relation merging should keep joining order

`joins_values.partition` will break joins values order. It should be
kept as user intended order.

Fixes #15488.

activerecord/lib/active_record/relation/merger.rb

@@ -112,22 +112,20 @@ module ActiveRecord
           if other.klass == relation.klass
             relation.joins!(*other.joins_values)
           else
-            joins_dependency, rest = other.joins_values.partition do |join|
+            alias_tracker = nil
+            joins_dependency = other.joins_values.map do |join|
               case join
               when Hash, Symbol, Array
-                true
+                alias_tracker ||= other.alias_tracker
+                ActiveRecord::Associations::JoinDependency.new(
+                  other.klass, other.table, join, alias_tracker
+                )
               else
-                false
+                join
               end
             end
 
-            join_dependency = ActiveRecord::Associations::JoinDependency.new(
-              other.klass, other.table, joins_dependency, other.alias_tracker
-            )
-
-            relation.joins! rest
-
-            @relation = relation.joins join_dependency
+            relation.joins!(*joins_dependency)
           end
         end
 

activerecord/test/cases/relation_test.rb

@@ -274,6 +274,15 @@ module ActiveRecord
       assert_equal({ 2 => 1, 4 => 3, 5 => 1 }, authors(:david).posts.merge(posts_with_special_comments_with_ratings).count)
     end
 
+    def test_relation_merging_keeps_joining_order
+      authors  = Author.where(id: 1)
+      posts    = Post.joins(:author).merge(authors)
+      comments = Comment.joins(:post).merge(posts)
+      ratings  = Rating.joins(:comment).merge(comments)
+
+      assert_equal 3, ratings.count
+    end
+
     class EnsureRoundTripTypeCasting < ActiveRecord::Type::Value
       def type
         :string