Add Truncate Almost All Tables as a rails til

rails/truncate-almost-all-tables.md

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+# 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]}
+```