don't truncate, return to known good state with specific test seeds

a.rb

# general support for dumping the post db:seed db to a hash
#


  def App.db_names
    result =
      Mongoid.session(:default).
        with(database: :admin).
          command({listDatabases:1})

    result['databases'].map{|hash| hash['name']}
  end
  
  def App.db_name
     Mongoid::Sessions.default.send(:current_database).name
  end
    
  def App.db_collections
    Mongoid::Sessions.default.collections
  end 
      
  def App.db_snapshot
    $db_collections = Map.new
    db_collections.each do |collection|
      name = collection.name
      next if name =~ /\bsystem\b|\$/
      documents = collection.find().to_a
      unless documents.blank? 
        $db_collections[name] = documents
      end
    end
    $db_collections
  end     

  def App.db_restore(snapshot = $db_collections)
    collections = db_collections
    
    (snapshot || Map.new).each do |name, documents|
      collection = collections.detect{|collection| collection.name == name}
      unless collection.blank? or documents.blank?
        collection.drop
        documents.each{|document| collection.insert(document)}
      end
    end
  end 



## due to lack of transactions in mongoid we snapshot the db before any test
# is run and then restore to this snapshot before each test *suite* is run.  we
# would like to restore before each test but it is simple too slow.  the side
# effect is that tests can pollute one another, but test suites cannot.  live
# with it.
#
  #App.db_snapshot()
  #at_exit{ App.db_restore() }
  test_db_yml = File.join(Rails.root, 'test/db.yml')
  abort('rake db:test:prepare ### did you forget?') unless test(?e, test_db_yml)
  TestDb = YAML.load(IO.read(test_db_yml))


  class MiniTest::Unit::TestCase
    before_all do
      App.db_restore(TestDb)
    end
  end