Post.joins(:users) should not be affected by `User.current_scope`

This change was introduced by #18109. The intent of that change was to
specifically apply `unscoped`, not to allow all changes to
`current_scope` to affect the join. The idea of allowing `current_scope`
to affect joins is interesting and potentially more consistent, but has
sever problems associated with it. The fact that we're specifically
stripping out joins indicates one such problem (and potentially leads to
invalid queries).

Ultimately it's difficult to reason about what `Posts.joins(:users)`
actually means if it's affected by `User.current_scope`, and it's
difficult to specifically control what does or doesn't get added. If we
were starting from scratch, I don't think I'd have `joins` be affected
by `default_scope` either, but that's too big of a breaking change to
make at this point.

With this change, we no longer apply `current_scope` when bringing in
joins, with the singular exception of the motivating use case which
introduced this bug, which is providing a way to *opt-out* of having the
default scope apply to joins.

Fixes #29338.

activerecord/CHANGELOG.md

+*   `Relation#joins` is no longer affected by the target model's
+    `current_scope`, with the exception of `unscoped`.
+
+    Fixes #29338.
+
+    *Sean Griffin*
+
 *   Change sqlite3 boolean serialization to use 1 and 0
 
     SQLite natively recognizes 1 and 0 as true and false, but does not natively

activerecord/lib/active_record/reflection.rb

@@ -219,10 +219,8 @@ def join_scopes(table, predicate_builder) # :nodoc:
       end
 
       def klass_join_scope(table, predicate_builder) # :nodoc:
-        if klass.current_scope
-          klass.current_scope.clone.tap { |scope|
-            scope.joins_values = scope.left_outer_joins_values = [].freeze
-          }
+        if klass.current_scope && klass.current_scope.values.empty?
+          klass.unscoped
         else
           klass.default_scoped(build_scope(table, predicate_builder))
         end

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -377,6 +377,16 @@ def test_default_scope_with_joins
                  Comment.joins(:post).count
   end
 
+  def test_joins_not_affected_by_scope_other_than_default_or_unscoped
+    without_scope_on_post = Comment.joins(:post).to_a
+    with_scope_on_post = nil
+    Post.where(id: [1, 5, 6]).scoping do
+      with_scope_on_post = Comment.joins(:post).to_a
+    end
+
+    assert_equal with_scope_on_post, without_scope_on_post
+  end
+
   def test_unscoped_with_joins_should_not_have_default_scope
     assert_equal SpecialPostWithDefaultScope.unscoped { Comment.joins(:special_post_with_default_scope).to_a },
                  Comment.joins(:post).to_a