相关疑难解决方法(0)

我收到以下错误:

NoMethodError in Users#new

Showing .../app/views/users/form/_new.haml where line #7 raised:

undefined method `on' for #<ActiveModel::Errors:0x007fb599ec6610>

第7行的代码是:

7:   = errors_for user, :first_name

和application_helper.rb:

def errors_for(model, field)
  error = case errors = model.errors.on(field)
  ...
end

'on'是ActiveRecord中的默认方法.为什么这不起作用？