undefined method ip for nil Class

I am trying to have a model Method return information from an associated model. It works without any issues on the Rails console and also outputs the information on the console when it's running as the web server.

To me it doesn't seem very difficult. But it's not working.

class Computer < ActiveRecord::Base
  has_many :ip_addresses
  has_one :status

  def first_ip
    @computer = Computer.find(self.id)
    @computer.ip_addresses.first.ip
    end
end


class IpAddress < ActiveRecord::Base
  attr_accessible :ip
  belongs_to :computers
end

[2011-10-10 16:09:02] ERROR ActionView::Template::Error: 开发者_StackOverflowundefined method `ip' for nil:NilClass
    /usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.10/lib/active_support/whiny_nil.rb:48:in `method_missing'
    /Users/robertg/RubymineProjects/CMDBTEST/app/models/computer.rb:7:in `first_ip'

Thanks

---

Do this:

class Computer < ActiveRecord::Base
  has_many :ip_addresses
  has_one :status

  def first_ip
    first_ip_address = ip_addresses.first
    first_ip_address ? first_ip_address.ip : nil
  end
end

Using your method, if the Computer has no ip_addresses, then calling .first will return nil, and there is no ip method for NilClass (just like the error says). This way, it checks if there are any ip_addresses, and if so returns the ip of the first ip_address, and if not returns nil.