Updated: Ruby: Don't build randomness into your factories

Changes

-Tests are about 100% control over UI interaction and your test scenario. Randomness makes writing tests hard. You will also push tests that are green for you today, but red for a colleague tomorrow.
+Tests are about 100% control over UI interaction and your test scenario. Randomness will eventually cause flaky tests that are hard to debug.
-That said, please don't do something like this:
+
+The case against random values
+------------------------------
+
+Avoid setting attributes to random values like this:
+
+```ruby
+factory(:document) do |document|
+  category { ['foo', 'bar', 'baz'].sample }
+end
+```
+
+This makes tests randomly fail when they expect "foo" for some other reason, but an unreleated document in the "foo" category happens to also be on the screen.
+
+Instead just use the same attribute value every time:
```ruby
-Factory(:document) do |document|
-document.category { ['foo', 'bar', 'baz'].sample }
+factory(:document) do |document|
+  category 'foo'
end
```
-Instead do this:
-
+### If you must use random values
+
+If you absolutely cannot use the same attribute value for every record, use a [FactoryBot sequence](https://thoughtbot.github.io/factory_bot/ref/sequence.html) and reset if before every example:
+
+```ruby
+factory(:document) do |document|
+  sequence(:category) { |i| ['foo', 'bar', 'baz'][i % 3] }
+end
+```
+
```ruby
-Factory(:document) do |document|
-document.category 'foo'
+# spec/support/factory_bot.rb
+
+RSpec.configure do |config|
+config.prepend_before do
+    FactoryBot.rewind_sequences
+  end
end
```
+
+
The case against Faker
----------------------
-I even recommend to **not** use libraries like [Faker](https://github.com/faker-ruby/faker). Faker makes awesome sample data, but at the risk of adding random strings to your screen.When your factory requires unique values for something, prefer numbering fixed strings intead:
+I even recommend to **not** use libraries like [Faker](https://github.com/faker-ruby/faker). Faker makes awesome sample data, but at the risk of adding random strings to your screen.
+
+When your factory requires unique values for something, prefer numbering fixed strings intead:
```ruby
FactoryBot.define do
  sequence(:first_name)   { |i| "First#{i}" }
  sequence(:last_name)    { |i| "Last#{i}" }
  sequence(:company_name) { |i| "Company #{i}" }
end
```
-This creates ugly sample data like "First3 Last3", but e.g. doesn't make tests pass when they shouldn't.
+This creates ugly sample data like "First3 Last3", but e.g. doesn't make tests pass when they shouldn't.
+
+
+### If you must use Faker
+
+If you must use Faker (e.g. you inherited a large test suite), at least use the same random seed for every test:
+
+```
+# spec/support/faker.rb
+
+RSpec.configure do |config|
+  config.prepend_before do
+    Faker::Config.random = Random.new(42)
+    Faker::UniqueGenerator.clear
+  end
+end
+```
+