NoMethodError in Users#new

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

undefined method `on' for #<ActiveModel::Errors:0x007fb599ec6610>@H_502_3@ 
 
第7行中的代码是：
 
  
 
7:   = errors_for user,:first_name@H_502_3@ 
 
而application_helper.rb：
 
  
 
def errors_for(model,field)
  error = case errors = model.errors.on(field)
  ...
end@H_502_3@ 
 
‘on’是一个default method in ActiveRecord.为什么这不工作？