Rails 3 - richtige test für die Methode mit rspec

Habe ich Modell TeacherLeader hält fremden Schlüssel für andere Tabellen. Ich kann eine Aktualisierung dieser Tabelle und ich versuche zu schreiben tests für meinen controller (via rspec).

Mein Modell:

#  id         :integer         not null, primary key
#  user_id    :integer
#  teacher_id :integer
#

class TeacherLeader < ActiveRecord::Base  
  belongs_to :user
  belongs_to :teacher

  validates :teacher_id, 
              :uniqueness => { :message => "already a class-head" }
end

Mein controller

class TeacherLeadersController < ApplicationController
 def edit
    @teacher_leader = TeacherLeader.find( params[:id] )    
    ...
  end

  def update
    @teacher_leader = TeacherLeader.find( params[:id] )
    ...

    if ( @teacher_leader.update_attributes( params[:teacher_leader] ) )                   
      redirect_to teachers_path
      ...
    else
      redirect_to edit_teacher_leader_path
      ...
      end      
    end
  end
end

So, ich habe Probleme mit dem schreiben von tests für. Hier ist zum Beispiel mein code für die PUT-Methode:

describe TeacherLeadersController do
  render_views

  before(:each) do    
    @teacher = Factory( :teacher )
    @teacher.user.user_role = "teacher"
    @teacher.save!

    @user = Factory( :user, :user_login => Factory.next(:user_login) )
    @user.user_role = "class_head"
    @user.save!

    @sh = Factory( :user, :user_login => Factory.next(:user_login)  )
    @sh.user_role = "school_head"
    @sh.save!
  end

  describe "PUT 'update" do
    before(:each) do
      @teacher_leader = @user.create_teacher_leader( @attr_teacher_leader )
    end

    describe "for signed-in school-heads" do
      before(:each) do
        test_sign_in( @sh )
      end

      it "should update teacher leader with correct params" do
        put :update, :id => @teacher_leader, :teacher_id => @teacher.id
        @teacher_leader.reload
        @teacher_leader.teacher_id.should  == @teacher.id
      end
    end
  end
end

Wenn ich versuche zu laufen, meine Angaben habe ich bekommen:

1) TeacherLeadersController PUT 'update for signed-in school-heads should update teacher leader with correct params
     Failure/Error: put :update, :id => @teacher_leader, :teacher_id => @teacher.id
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./app/controllers/teacher_leaders_controller.rb:57:in `update'
     # ./spec/controllers/teacher_leaders_controller_spec.rb:150:in `block (4 levels) in <top (required)>'

Aber mein Programm funktioniert! Was mache ich in tests falsch?