Add All or Nothing Database Transactions as a rails til.

rails/all-or-nothing-database-transactions.md

@@ -0,0 +1,22 @@
+# All or Nothing Database Transactions
+
+When you are updating multiple records in an *all or nothing* scenario, you
+can use [Active Record
+Transactions](http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html)
+to ensure that either everything is updated or none of it is updated.
+
+For instance, if you are transferring *internet points* from one user's
+account to another user's account, you need to be sure that the transfer
+balances out. If updating one user is successful, but updating the other
+fails, then there will be a discrepancy in the data. A transaction will
+ensure that when any part of the update fails the entire transaction is
+rolled back (at the database level).
+
+```ruby
+User.transaction do
+  user1.internet_points += 20
+  user2.internet_points -= 20
+  user1.save!
+  user2.save!
+end
+```