Add Truncate Almost All Tables as a rails til

README.md

@@ -229,6 +229,7 @@ _351 TILs and counting..._
 - [Show Pending Migrations](rails/show-pending-migrations.md)
 - [Show Rails Models With Pry](rails/show-rails-models-with-pry.md)
 - [Show Rails Routes With Pry](rails/show-rails-routes-with-pry.md)
+- [Truncate Almost All Tables](rails/truncate-almost-all-tables.md)
 
 ### Ruby
 

rails/truncate-almost-all-tables.md

@@ -0,0 +1,19 @@
+# Truncate Almost All Tables
+
+The
+[`database_cleaner`](https://github.com/DatabaseCleaner/database_cleaner)
+gem is a handy way to make sure you have a consistent database context for
+each test example or suite. One `database_cleaner` strategy that can be used
+is the `truncation` strategy. This truncates the data from all the tables by
+default. This is not ideal for *fixed* tables that contain domain-specific
+data because you end up having to do way more test setup than should be
+necessary. Fortunately, specific tables can be excepted by the truncation
+strategy using the `except` option.
+
+For instance, if we have a standard set of roles for users of our
+application, we can except that table from truncation with a line like the
+following in our `rails_helper.rb` file:
+
+```
+DatabaseCleaner.strategy = :truncation, {:except => %w[roles]}
+```