Undefined 'name' for nil:NilClass

con.rb

class Con < ActiveRecord::Base
  validates_presence_of :name
  validates_uniqueness_of :name

  after_create :create_tag
  after_save :rename_tag

  def create_tag
    tag = Tag.new(:name => self.name, :tag_group => TagGroup.find_by_name('Cons'))
    tag.save
  end

  def rename_tag
    tag = Tag.find_by_group_and_tag_name('Cons', self.name)
    puts tag

    name = tag.name
    puts name
  end
end

cons_controller.rb

class ConsController < ApplicationController
  def create
    con = Con.new(params[:con])

    if con.save!
      redirect_to con, :notice => "Con created successfully"
    end
  end
end

cons_controller_test.rb

require 'test_helper'

class ConsControllerTest < ActionController::TestCase

  context "with a TagGroup called 'Cons'" do
    setup do
      Factory :tag_group, :name => "Cons"
    end

    should "create a tag when created" do
      assert_difference('Tag.count') do
        post :create, :con => {:name => "New Con"}
      end

      tag = Tag.find_by_name("New Con")
      assert_not_nil tag
      assert_equal TagGroup.find_by_name('Cons'), tag.tag_group
    end

    #### THE OFFENDING TASK ####
    should "rename its tag when renamed" do
      con = Factory(:con, :name=>"New Con")
      tag = Tag.find_by_name(con.name)

      assert_equal "New Con", tag.name

      con.name = "Changed Name"
      con.save

      #assert_equal "Changed Name", tag.name
    end

    should "not remove its tag when deleted" do
      assert false
    end
  end

end

Output

#<Tag:0x00000005002fd0>
New Con


NoMethodError: undefined method `name' for nil:NilClass
/home/lsimons/RubymineProjects/connie/app/models/con.rb:16:in `rename_tag'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:405:in `_run__1766852513517797900__save__1160347338664056617__callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:405:in `__run_callback'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:385:in `_run_save_callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:81:in `run_callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/callbacks.rb:264:in `create_or_update'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/persistence.rb:84:in `save'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/validations.rb:50:in `save'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/attribute_methods/dirty.rb:22:in `save'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:241:in `block (2 levels) in save'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:295:in `block in with_transaction_returning_status'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/connection_adapters/abstract/database_statements.rb:192:in `transaction'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:208:in `transaction'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:241:in `block in save'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:252:in `rollback_active_record_state!'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activerecord-3.2.2/lib/active_record/transactions.rb:240:in `save'
/home/lsimons/RubymineProjects/connie/test/functional/cons_controller_test.rb:27:in `block (2 levels) in <class:ConsControllerTest>'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/shoulda-context-1.0.0/lib/shoulda/context/context.rb:398:in `call'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/shoulda-context-1.0.0/lib/shoulda/context/context.rb:398:in `block in create_test_from_should_hash'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:1046:in `run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/testing/setup_and_teardown.rb:35:in `block in run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:458:in `_run__574566149628552421__setup__1160347338664056617__callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:405:in `__run_callback'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:385:in `_run_setup_callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:81:in `run_callbacks'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/activesupport-3.2.2/lib/active_support/testing/setup_and_teardown.rb:34:in `run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/test_runner.rb:21:in `run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:68:in `_run_test'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:55:in `block in _run_suite'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:55:in `each'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:55:in `_run_suite'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:40:in `block (2 levels) in _run_anything'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:40:in `each'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:40:in `block in _run_anything'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:106:in `fix_sync'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-reporters-0.5.1/lib/minitest/suite_runner.rb:39:in `_run_anything'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:952:in `run_tests'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:939:in `block in _run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:938:in `each'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:938:in `_run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:927:in `run'
/home/lsimons/.rvm/gems/ruby-1.9.2-p290/gems/minitest-2.11.3/lib/minitest/unit.rb:695:in `block in autorun'