Rails 4.2 uniquenness.rb:55 in find_finder_class_for error undefined method `abstract_class?'对于零：NilClass

如何解决Rails 4.2 uniquenness.rb:55 in find_finder_class_for error undefined method `abstract_class?'对于零：NilClass

我正在将 rails 从 v4.0.13 更新到 v4.2.11.3，当我要保存某个对象时，出现以下错误：

导轨控制台： User.new.save

错误： NoMethodError: undefined method abstract_class?' for nil:NilClass from /.rbenv/versions/2.6.6/lib/ruby/gems/2.6.0/gems/activerecord-4.2.11.3/lib/active_record/validations/uniqueness.rb:55:in block in find_finder_class_for'

这是https://github.com/rails/rails/blob/main/activerecord/lib/active_record/validations/uniqueness.rb中的方法：

     def find_finder_class_for(record)
       class_hierarchy = [record.class]

       while class_hierarchy.first != @klass
         class_hierarchy.unshift(class_hierarchy.first.superclass)
       end

       class_hierarchy.detect { |klass| !klass.abstract_class? }
     end

有人帮我吗？