```ruby
require 'test_helper'

class RoundTest < ActiveSupport::TestCase
  test "newly created round has randomly generated url string" do
    r = Round.new
    assert_not_nil r.url
    assert_match /^[[:xdigit:]]{6}$/, r.url

    r2 = Round.new
    assert_not_equal r.url, r2.url
  end

  test "newly created round has expire time of one hour" do
    r = Round.new
    assert_not_nil r.expire_time
    assert Time.now + 1.hours - r.expire_time < 1 # fuzzy time comparison
  end

  test "url and expire_time persist on save" do
    r = Round.new
    url = r.url
    r.save

    r2 = Round.where(url: url).first

    assert_equal r2.url, r.url
    assert_equal r2.expire_time, r.expire_time
  end
end
```

Test Round.seconds_left returns time until round expires
user
```ruby
require 'test_helper'

class RoundTest < ActiveSupport::TestCase
  test "newly created round has randomly generated url string" do
    r = Round.new
    assert_not_nil r.url
    assert_match /^[[:xdigit:]]{6}$/, r.url

    r2 = Round.new
    assert_not_equal r.url, r2.url
  end

  test "newly created round has expire time of one hour" do
    r = Round.new
    assert_not_nil r.expire_time
    assert Time.now + 1.hours - r.expire_time < 1 # fuzzy time comparison
  end

  test "url and expire_time persist on save" do
    r = Round.new
    url = r.url
    r.save

    r2 = Round.where(url: url).first

    assert_equal r2.url, r.url
    assert_equal r2.expire_time, r.expire_time
  end

  test "seconds_left returns time until round expires" do
    r = Round.new

    now = Time.now
    r.expire_time = now + 1.hours

    assert r.seconds_left - 1.hours < 1
  end
end
```
assistant