Add Mask An ActiveRecord Attribute as a Rails til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1104 TILs and counting..._
+_1105 TILs and counting..._
 
 ---
 
@@ -694,6 +694,7 @@ _1104 TILs and counting..._
 - [Make ActionMailer Synchronous In Test](rails/make-action-mailer-synchronous-in-test.md)
 - [Manually Run A Migration From Rails Console](rails/manually-run-a-migration-from-rails-console.md)
 - [Mark For Destruction](rails/mark-for-destruction.md)
+- [Mask An ActiveRecord Attribute](rails/mask-an-activerecord-attribute.md)
 - [Merge A Scope Into An ActiveRecord Query](rails/merge-a-scope-into-an-activerecord-query.md)
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)
 - [Order Matters For `rescue_from` Blocks](rails/order-matters-for-rescue-from-blocks.md)

rails/mask-an-activerecord-attribute.md

@@ -0,0 +1,39 @@
+# Mask An ActiveRecord Attribute
+
+Let's say we have a `User` model with backing table that has an `email`
+attribute.
+
+If we look up a `User` record, we can grab its email because Rails provides an
+accessor to that attribute under the hood.
+
+```ruby
+> user.email
+'Liz.Lemon@example.com'
+```
+
+We can write a custom `#email` method on `User` to mask that attribute. We
+could do this for any number of reasons. One might be to always downcase the
+email before retrieving it and using it througout app code.
+
+```ruby
+class User < ApplicationRecord
+  def email
+    read_attribute(:email).downcase
+  end
+end
+```
+
+This uses the
+[`#read_attribute`](https://api.rubyonrails.org/classes/ActiveRecord/AttributeMethods/Read.html#method-i-read_attribute)
+method which retreives the type casted value before hitting the accessor
+method. We have to do this, rather than calling `#email` directly, because that
+would result in an infinite loop.
+
+```ruby
+> user.email
+'liz.lemon@example.com'
+```
+
+I'd probably handle this email scenario at the DB-layer. Nevertheless, this
+demonstrates a technique we can use in a variety of scenarios at the
+Rails-layer.