Why do I get an assert but all test are passing - ruby-on-rails

Question

need some help, I run "bundle exec rake" on a new rails application and I am getting an assertion, yet my test has passed, I can see in the test.log that my controller received a 200 OK response.
My app is basic:
About your application's environment
Rails version 4.2.0
Ruby version 2.2.0-p0 (x86_64-linux)
RubyGems version 2.4.5
Rack version 1.5
JavaScript Runtime Node.js (V8)
Middleware Rack::Sendfile, ActionDispatch::Static, Rack::Lock, #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x00000002835098>, Rack::Runtime, Rack::MethodOverride, ActionDispatch::RequestId, Rails::Rack::Logger, ActionDispatch::ShowExceptions, ActionDispatch::DebugExceptions, ActionDispatch::RemoteIp, ActionDispatch::Reloader, ActionDispatch::Callbacks, ActiveRecord::Migration::CheckPending, ActiveRecord::ConnectionAdapters::ConnectionManagement, ActiveRecord::QueryCache, ActionDispatch::Cookies, ActionDispatch::Session::CookieStore, ActionDispatch::Flash, ActionDispatch::ParamsParser, Rack::Head, Rack::ConditionalGet, Rack::ETag
Application root /home/ykoehler/work/test2
Environment development
Database adapter postgresql
Database schema version 0
After having created my app I did
rake db:create db:migrate
rails g controller About index
rake test
Here is the output:
Run options: --seed 52388
# Running:
.
Finished in 0.049576s, 20.1709 runs/s, 20.1709 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
and the test.log
ActiveRecord::SchemaMigration Load (0.5ms) SELECT "schema_migrations".* FROM "schema_migrations"
(106.2ms) DROP DATABASE IF EXISTS "test2_test"
(308.1ms) CREATE DATABASE "test2_test" ENCODING = 'unicode'
SQL (0.5ms) CREATE EXTENSION IF NOT EXISTS "plpgsql"
(5.9ms) CREATE TABLE "schema_migrations" ("version" character varying NOT NULL)
(3.0ms) CREATE UNIQUE INDEX "unique_schema_migrations" ON "schema_migrations" ("version")
(0.6ms) SELECT version FROM "schema_migrations"
(1.1ms) INSERT INTO "schema_migrations" (version) VALUES ('0')
ActiveRecord::SchemaMigration Load (0.4ms) SELECT "schema_migrations".* FROM "schema_migrations"
(1.3ms) BEGIN
------------------------------------------
AboutControllerTest: test_should_get_index
------------------------------------------
Processing by AboutController#index as HTML
Rendered about/index.html.erb within layouts/application (0.8ms)
Completed 200 OK in 14ms (Views: 13.9ms | ActiveRecord: 0.0ms)
(0.4ms) ROLLBACK
As you can see it says "Completed 200 OK" and test is outputted with a "." which means success. But I still get an "1 assertions", I do not understand why and where it come from.

Answer 1

Your result:
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
Understanding this:
1 runs: How many times the test was run? Here 1.
1 assertions: An assertion is a line of code that evaluates an object (or expression) for expected results. So, How many times assertion was called? Answer 1.
0 failures: How many times assertion failed? Here none.
0 errors: How many errors were raised? (may be syntax, logic, runtime..). Here none.
0 skips: There are a few callbacks that you dont want to call while running tests (like sending mails etc) as these add no value to test and in some case slows down the process. You can skip these callbacks. So, how many skips were made? Here none.