STI cast new instances to `default type` on initialize.

fixes #17121

1 files changed, 12 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 8b719e0bcb..ed95b30f80 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -55,6 +55,8 @@ module ActiveRecord
           subclass = subclass_from_attributes(attrs)
         end
 
+        subclass ||= subclass_from_defaults
+
         if subclass && subclass != self
           subclass.new(*args, &block)
         else
@@ -201,6 +203,16 @@ module ActiveRecord
         attribute_names.include?(inheritance_column) && (attrs.is_a?(Hash) || attrs.respond_to?(:permitted?))
       end
 
+      def subclass_from_defaults?
+        attribute_names.include?(inheritance_column) && columns_hash[inheritance_column].try(:default)
+      end
+
+      def subclass_from_defaults
+        if subclass_from_defaults?
+          find_sti_class(columns_hash[inheritance_column].default)
+        end
+      end
+
       def subclass_from_attributes(attrs)
         attrs = attrs.to_h if attrs.respond_to?(:permitted?)
         subclass_name = attrs.with_indifferent_access[inheritance_column]