irb

CRuby 1.8.7 >> Switch.first
CRuby 1.8.7 => #<Switch ip_address=#<IPAddr: IPv4:192.168.1.240/255.255.255.255> name=nil>
CRuby 1.8.7 >> Port.first
CRuby 1.8.7 => #<Port trunk=false port_number=1 switch_ip_address=#<IPAddr: IPv4:192.168.1.240/255.255.255.255>>
CRuby 1.8.7 >> Switch.first.ports
NoMethodError: undefined method `ports_association' for nil:NilClass
        from /home/konstantin/.config/irb/myirb.rb:108:in `method_missing'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:108:in `send'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:108:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:107:in `each'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:107:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:174:in `with_repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/support/kernel.rb:6:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core.rb:181:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/repository.rb:44:in `scope'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core.rb:181:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/support/kernel.rb:6:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:174:in `with_repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/relationship.rb:82:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/one_to_many.rb:251:in `children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.10/lib/dm-core/associations/one_to_many.rb:309:in `method_missing'
        from /usr/lib/ruby/1.8/irb.rb:302:in `orig_output_value'
... 8 levels...
        from /usr/lib/ruby/1.8/irb/ruby-lex.rb:244:in `each_top_level_statement'
        from /usr/lib/ruby/1.8/irb/ruby-lex.rb:230:in `loop'
        from /usr/lib/ruby/1.8/irb/ruby-lex.rb:230:in `each_top_level_statement'
        from /usr/lib/ruby/1.8/irb/ruby-lex.rb:229:in `catch'
        from /usr/lib/ruby/1.8/irb/ruby-lex.rb:229:in `each_top_level_statement'
        from /usr/lib/ruby/1.8/irb.rb:146:in `eval_input'
        from /usr/lib/ruby/1.8/irb.rb:70:in `start'
        from /usr/lib/ruby/1.8/irb.rb:69:in `catch'
        from /usr/lib/ruby/1.8/irb.rb:69:in `start'
        from /usr/lib/ruby/gems/1.8/gems/merb-core-1.0.9/lib/merb-core/rack/adapter/irb.rb:161:in `start'
        from /usr/lib/ruby/gems/1.8/gems/merb-core-1.0.9/lib/merb-core/server.rb:174:in `bootup'
        from /usr/lib/ruby/gems/1.8/gems/merb-core-1.0.9/lib/merb-core/server.rb:42:in `start'
        from /usr/lib/ruby/gems/1.8/gems/merb-core-1.0.9/lib/merb-core.rb:170:in `start'
        from /usr/lib/ruby/gems/1.8/gems/merb-core-1.0.9/bin/merb:11
        from /usr/bin/merb:19:in `load'
        from /usr/bin/merb:19Maybe IRB bug!!

port.rb

class Port
  include DataMapper::Resource

  property :trunk, Boolean, :default => false
  property :port_number, Integer, :key => true
  property :switch_ip_address, IPAddress, :key => true

  belongs_to :switch
  has n, :connections

end

switch.rb

class Switch
  include DataMapper::Resource

  has n, :ports
  property :ip_address, IPAddress, :key => true
  property :name, String

  def snmp(&block)
    SNMP::Manager.open({ :Host => ip_address.to_s }, &block)
  end

  def snmp_get(name)
    snmp { |s| s.get_value(name) }
  end

end