Custom validations causing errors - ruby-on-rails

Question

I am new to rails and am following the Depot application in the Pragmatic Agile Web development with rails and I am having an odd problem.
In my product model I created a validator for confirming that the image asked for in the image url field actually exist as an asset. Here is my product model code.
class Product < ActiveRecord::Base
attr_accessible :description, :image_url, :price, :title
validates :description, :price, :title, :presence => true
validate :image_url_exists, on: :create
def image_url_exists
if Rails.application.assets.find_asset(image_url) == nil
errors.add(:image_url, 'is not valid. The image does not exist.')
end
end
end
Now the problem is when I run my unit test. Here is what it is:
require 'test_helper'
class ProductTest < ActiveSupport::TestCase
test "the products attributes should not be empty" do
p = Product.new
assert p.invalid?
end
end
But doing so triggers a bunch of errors in my code. Without the custom validtor, everything seems to work just fine. Here are the errors I am currently getting.
test_the_products_attributes_should_not_be_empty(ProductTest):
TypeError: can't convert nil into String
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/base.rb:156:in `initialize'
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/base.rb:156:in `new'
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/base.rb:156:in `find_asset'
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/index.rb:60:in `find_asset'
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/environment.rb:78:in `find_asset'
/home/saurabh/Desktop/SCRIPTS/Rails/depot/app/models/product.rb:6:in `image_url_exists'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:418:in `_run__4343689776242734370__validate__107120755283260520__callbacks'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:405:in `__run_callback'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:385:in `_run_validate_callbacks'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:81:in `run_callbacks'
/var/lib/gems/1.9.1/gems/activemodel-3.2.9/lib/active_model/validations.rb:228:in `run_validations!'
/var/lib/gems/1.9.1/gems/activemodel-3.2.9/lib/active_model/validations/callbacks.rb:53:in `block in run_validations!'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:403:in `_run__4343689776242734370__validation__107120755283260520__callbacks'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:405:in `__run_callback'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:385:in `_run_validation_callbacks'
/var/lib/gems/1.9.1/gems/activesupport-3.2.9/lib/active_support/callbacks.rb:81:in `run_callbacks'
/var/lib/gems/1.9.1/gems/activemodel-3.2.9/lib/active_model/validations/callbacks.rb:53:in `run_validations!'
/var/lib/gems/1.9.1/gems/activemodel-3.2.9/lib/active_model/validations.rb:195:in `valid?'
/var/lib/gems/1.9.1/gems/activerecord-3.2.9/lib/active_record/validations.rb:69:in `valid?'
/var/lib/gems/1.9.1/gems/activemodel-3.2.9/lib/active_model/validations.rb:203:in `invalid?'

Answer 1

The problem is Rails.application.assets.find_asset(image_url), image_url is nil when you create a refresh object at test:
p = Product.new # Here p.image_url => nil
You can find the reason of the error looking at log
/var/lib/gems/1.9.1/gems/sprockets-2.2.2/lib/sprockets/base.rb:156:in `initialize'
Here (link) the line 156 where error happens, path was nil here.
So to solve this problem you need to add a condition at your if to check if image_url isn't nil, here is my sugestion
if self.image_url.nil? || Rails.application.assets.find_asset(self.image_url).nil?
errors.add(:image_url, 'is not valid. The image does not exist.')
end