Sequelを使ったときのテストがうまく書けない

.gitignore

memo.db

memo.rb

require 'rubygems'
require 'sequel'

Sequel::Model.plugin(:schema)
Sequel.connect("sqlite://memo.db")

class Memo < Sequel::Model
  unless table_exists?
    set_schema do
      primary_key :id
      string :memo_id, :unique => true, :null => false
      text :body
    end
    create_table
  end

  def self.make_id
    Time.now.to_i.to_s(36).reverse
  end

  def before_create
    self.memo_id ||= make_id
  end
end

def create_memo(body, memo_id=Memo.make_id)
  values = {
    :body => body,
    :memo_id => memo_id,
  }
  begin
    retry_count = 0
    Memo.create(values)
  rescue Sequel::DatabaseError => e
    if /memo_id is not unique/ =~ e.to_s && retry_count < 3
      values[:memo_id].succ!
      retry
    end
    raise e
  end
end

memo_test.rb

#!/usr/bin/ruby -Ku
require 'test/unit'
require 'memo'

class PasteTest < Test::Unit::TestCase

  TEST_DB = "test.db"
  def setup
    @db = Sequel.connect("sqlite://#{TEST_DB}")
  end

  def teardown
    @db.disconnect
    File.unlink(TEST_DB) if File.exist?(TEST_DB)
  end

  def test_memo_body
    memo = create_memo("foo", "foo")
    assert_equal("foo", memo.body)
    memo = create_memo("bar", "foo")
    assert_equal("bar", memo.body)
  end

  def test_memo
    memo = create_memo("foo", "foo")
    assert_equal("foo", memo.memo_id)
    memo = create_memo("foo", "foo")
    assert_equal("fop", memo.memo_id)
  end
end