Do not use default attributes for STI when instantiating a subclass

The commit which originally added this behavior did not consider that
doing `Subclass.new` does not actually populate the `type` field in the
attributes (though perhaps it should). We simply need to not use the
defaults for STI related things unless we are instantiating the base
class.

Fixes #23285.

2 files changed, 9 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 3a17f74b1d..3b6fb70d0d 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -52,7 +52,11 @@ module ActiveRecord
 
         attrs = args.first
         if has_attribute?(inheritance_column)
-          subclass = subclass_from_attributes(attrs) || subclass_from_attributes(column_defaults)
+          subclass = subclass_from_attributes(attrs)
+
+          if subclass.nil? && base_class == self
+            subclass = subclass_from_attributes(column_defaults)
+          end
         end
 
         if subclass && subclass != self
diff --git a/activerecord/test/cases/inheritance_test.rb b/activerecord/test/cases/inheritance_test.rb
index c870247a4a..7da6842047 100644
--- a/activerecord/test/cases/inheritance_test.rb
+++ b/activerecord/test/cases/inheritance_test.rb
@@ -493,6 +493,10 @@ class InheritanceComputeTypeTest < ActiveRecord::TestCase
     assert_equal 'Firm', firm.type
     assert_instance_of Firm, firm
 
+    client = Client.new
+    assert_equal 'Client', client.type
+    assert_instance_of Client, client
+
     firm = Company.new(type: 'Client') # overwrite the default type
     assert_equal 'Client', firm.type
     assert_instance_of Client, firm