[Rails] issues with model in ruby on rails

*class SystemAdmin < ActiveRecord::Base*
* belongs_to :user*
*end*
*
*
*class User < ActiveRecord::Base*
* has_secure_password*
* *
* has_one :system_admin *
*end*
*
*
*@user = User.find_by_user_name(user_name)*
* if !@user.system_admins.nil?*
* puts 'am a sys_admin'*
*
*
The above code gives error like this
*
*
NoMethodError (undefined method `system_admins' for #<User:0xa81a2e0>):
app/controllers/application_controller.rb:17:in `user_authenticate'

Have any problem with this condition checking* if
!@user.system_admins.nil? *also any problem with the model relation?
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*Thank you*
*vishnu*
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