don't rely on the columns hash to get defaults. follow-up to #17169.

This will also get the defaults from attribute definitions like:

     attribute :type, :string, default: "SomethingElse"

activerecord/lib/active_record/inheritance.rb

@@ -52,7 +52,7 @@ module ActiveRecord
 
         attrs = args.first
         if has_attribute?(inheritance_column)
-          subclass = subclass_from_attributes(attrs) || subclass_from_defaults
+          subclass = subclass_from_attributes(attrs) || subclass_from_attributes(column_defaults)
         end
 
         if subclass && subclass != self
@@ -207,12 +207,6 @@ module ActiveRecord
           end
         end
       end
-
-      def subclass_from_defaults
-        if default = columns_hash[inheritance_column].default
-          find_sti_class(default)
-        end
-      end
     end
 
     def initialize_dup(other)

activerecord/test/cases/inheritance_test.rb

@@ -500,3 +500,27 @@ class InheritanceComputeTypeTest < ActiveRecord::TestCase
     Company.reset_column_information
   end
 end
+
+class InheritanceAttributeTest < ActiveRecord::TestCase
+
+  class Company < ActiveRecord::Base
+    self.table_name = 'companies'
+    attribute :type, :string, default: "InheritanceAttributeTest::Startup"
+  end
+
+  class Startup < Company
+  end
+
+  class Empire < Company
+  end
+
+  def test_inheritance_new_with_subclass_as_default
+    startup = Company.new # without arguments
+    assert_equal 'InheritanceAttributeTest::Startup', startup.type
+    assert_instance_of Startup, startup
+
+    empire = Company.new(type: 'InheritanceAttributeTest::Empire') # without arguments
+    assert_equal 'InheritanceAttributeTest::Empire', empire.type
+    assert_instance_of Empire, empire
+  end
+end