Stash `left_joins` into `joins` to deduplicate redundant LEFT JOIN

Originally the `JoinDependency` has the deduplication for eager loading
(LEFT JOIN). This re-uses that deduplication for `left_joins`.

And also, This makes left join order into part of joins, i.e.:

Before:

```
association joins -> stash joins (eager loading, etc) -> string joins -> left joins
```

After:

```
association joins -> stash joins (eager loading, left joins, etc) -> string joins
```

Now string joins are able to refer left joins.

Fixes #34325.
Fixes #34332.
Fixes #34536.

1 files changed, 7 insertions, 16 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 4de7465128..6bb77b355c 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -117,16 +117,14 @@ module ActiveRecord
           if other.klass == relation.klass
             relation.joins!(*other.joins_values)
           else
-            joins_dependency = other.joins_values.map do |join|
+            associations, others = other.joins_values.partition do |join|
               case join
-              when Hash, Symbol, Array
-                other.send(:construct_join_dependency, join)
-              else
-                join
+              when Hash, Symbol, Array; true
               end
             end
 
-            relation.joins!(*joins_dependency)
+            join_dependency = other.construct_join_dependency(associations)
+            relation.joins!(join_dependency, *others)
           end
         end
 
@@ -136,16 +134,9 @@ module ActiveRecord
           if other.klass == relation.klass
             relation.left_outer_joins!(*other.left_outer_joins_values)
           else
-            joins_dependency = other.left_outer_joins_values.map do |join|
-              case join
-              when Hash, Symbol, Array
-                other.send(:construct_join_dependency, join)
-              else
-                join
-              end
-            end
-
-            relation.left_outer_joins!(*joins_dependency)
+            associations = other.left_outer_joins_values
+            join_dependency = other.construct_join_dependency(associations)
+            relation.joins!(join_dependency)
           end
         end