Relation merging should keep joining order

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

Fixes #15488.

1 files changed, 8 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index ebc72d28fd..b736b21525 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/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