Keep INNER JOIN when merging relations

Doing `Author.joins(:posts).merge(Post.joins(:comments))` does this
`SELECT ... INNER JOIN posts ON... LEFT OUTER JOIN comments ON...`
instead of doing
`SELECT ... INNER JOIN posts ON... INNER JOIN comments ON...`.

This behavior is unexpected and makes little sense as, basically, doing
`Post.joins(:comments)` means I want posts that have comments. Turning
it to a LEFT JOIN means I want posts and join the comments data, if
any.

We can see this problem directly in the existing tests.
The test_relation_merging_with_merged_joins_as_symbols only does joins
from posts to comments to ratings while the ratings fixture isn't
loaded, but the count is non-zero.

1 files changed, 3 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index 643226267c..83e5c23cc4 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -104,17 +104,17 @@ module ActiveRecord
         join_root.drop(1).map!(&:reflection)
       end
 
-      def join_constraints(outer_joins, join_type)
+      def join_constraints(joins_to_add, join_type)
         joins = join_root.children.flat_map { |child|
           make_join_constraints(join_root, child, join_type)
         }
 
-        joins.concat outer_joins.flat_map { |oj|
+        joins.concat joins_to_add.flat_map { |oj|
           if join_root.match? oj.join_root
             walk join_root, oj.join_root
           else
             oj.join_root.children.flat_map { |child|
-              make_outer_joins oj.join_root, child
+              make_join_constraints(oj.join_root, child, join_type)
             }
           end
         }