Topic: rake test ignores fixtures?

i have a model (simplified)

    class Myfile < ActiveRecord::Base
      validates_attachment_size :body, :less_than => AdminOptions.first.max_file_size.megabytes
    end

max_file_size is defined in AdminOptions fixtures, but when i try to run "rake test", i get:

     /usr/local/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/whiny_nil.rb:52:in `method_missing': undefined method `max_file_size' for nil:NilClass (NoMethodError)
     from /myapp/app/models/myfile.rb:1

if i try to run one of the tests separately, like

    ruby test/unit/myfile_test.rb

i get same mistake. 
but if i load fixtures before:

    RAILS_ENV=test rake db:load:fixtures
    ruby test/unit/myfile_test.rb

tests execute properly.

if i try to load fixtures before executing all tests

    RAILS_ENV=test rake db:load:fixtures
    rake test

it fails with same mistake. 
does anyone know how to fix that?