Fix unscoping `default_scope` in STI associations

Since 5c71000d, it has lost to be able to unscope `default_scope` in STI
associations. This change will use `.empty_scope?` instead of
`.values.empty?` to regard as an empty scope if only have
`type_condition`.

activerecord/lib/active_record/reflection.rb

@@ -219,8 +219,10 @@ def join_scopes(table, predicate_builder) # :nodoc:
       end
 
       def klass_join_scope(table, predicate_builder) # :nodoc:
-        if klass.current_scope && klass.current_scope.values.empty?
-          klass.unscoped
+        current_scope = klass.current_scope
+
+        if current_scope && current_scope.empty_scope?
+          build_scope(table, predicate_builder)
         else
           klass.default_scoped(build_scope(table, predicate_builder))
         end

activerecord/lib/active_record/relation.rb

@@ -642,6 +642,10 @@ def inspect
       "#<#{self.class.name} [#{entries.join(', ')}]>"
     end
 
+    def empty_scope? # :nodoc:
+      @values == klass.unscoped.values
+    end
+
     protected
 
       def load_records(records)

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -392,6 +392,22 @@ def test_unscoped_with_joins_should_not_have_default_scope
                  Comment.joins(:post).to_a
   end
 
+  def test_sti_association_with_unscoped_not_affected_by_default_scope
+    post = posts(:thinking)
+    comments = [comments(:does_it_hurt)]
+
+    post.special_comments.update_all(deleted_at: Time.now)
+
+    assert_raises(ActiveRecord::RecordNotFound) { Post.joins(:special_comments).find(post.id) }
+    assert_equal [], post.special_comments
+
+    SpecialComment.unscoped do
+      assert_equal post, Post.joins(:special_comments).find(post.id)
+      assert_equal comments, Post.joins(:special_comments).find(post.id).special_comments
+      assert_equal comments, Post.eager_load(:special_comments).find(post.id).special_comments
+    end
+  end
+
   def test_default_scope_select_ignored_by_aggregations
     assert_equal DeveloperWithSelect.all.to_a.count, DeveloperWithSelect.count
   end

activerecord/test/models/comment.rb

@@ -54,6 +54,7 @@ def to_s
 end
 
 class SpecialComment < Comment
+  default_scope { where(deleted_at: nil) }
 end
 
 class SubSpecialComment < SpecialComment

activerecord/test/schema/schema.rb

@@ -189,6 +189,7 @@
     t.string :resource_id
     t.string :resource_type
     t.integer :developer_id
+    t.datetime :deleted_at
   end
 
   create_table :companies, force: true do |t|