asset.rb

class Asset < ActiveRecord::Base
  belongs_to :task
  validate :task_belongs_to_creator
  
  attr_accessor :creator
  
  private
  
  def task_belongs_to_creator
    if creator && task.user != creator
      errors.add(:task, "should belong to it's creator")
    end
  end
end

asset_test.rb

class AssetTest < Test::Unit::TestCase  

  context "An asset with a creator" do
    setup do
      @user = User.create!
      @asset = Asset.new(:creator => @user)
    end
    
    should "not be allowed to associate with tasks of another user" do
      another_user = User.create!
      
      @asset.task = another_user.tasks.create!

      @asset.valid?
      assert_match /should belong to it's creator/, @asset.errors[:task]
    end
    
    should "be allowed to associate with tasks of its creator" do
      @asset.task = @user.tasks.create!
      @asset.valid?
      
      assert_nil @asset.errors[:task]
    end    
  end
  
end

assets_controller.rb

class AssetsController  
  def create
    @asset = Asset.new(params[:asset].merge(:creator => current_user))
    @asset.save!
  end
end