Add Inspect Previous Changes To ActiveRecord Object as a rails til

Author: jbranchaud

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_949 TILs and counting..._
+_950 TILs and counting..._
 
 ---
 
@@ -589,6 +589,7 @@ _949 TILs and counting..._
 - [Get The Column Names For A Model](rails/get-the-column-names-for-a-model.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)
+- [Inspect Previous Changes To ActiveRecord Object](rails/inspect-previous-changes-to-activerecord-object.md)
 - [List The Enqueued Jobs](rails/list-the-enqueued-jobs.md)
 - [Log SQL Queries Executed By ActiveRecord](rails/log-sql-queries-executed-by-activerecord.md)
 - [Mark A Migration As Irreversible](rails/mark-a-migration-as-irreversible.md)

rails/inspect-previous-changes-to-activerecord-object.md

@@ -0,0 +1,33 @@
+# Inspect Previous Changes To ActiveRecord Object
+
+If you modify an ActiveRecord object, before saving it, you can inspect changes
+with methods like `changed?` and `<attr>_changed?`:
+
+```ruby
+book.title = "The Fifth Season"
+
+book.changed? #=> true
+book.title_changed? #=> true
+book.publication_year_changed? #=> false
+
+book.changes
+#=> { "title" => ["Original Title", "The Fifth Season"] }
+```
+
+After saving an object, it will no longer be in a _dirty_ state and these
+methods will have no _changes_ to return.
+
+If you have a reference to the saved ActiveRecord object, you can look at the
+_previous_ changes with methods like `previous_changes` and
+`<attr>_previously_changed?`:
+
+```ruby
+book.title = "The Fifth Season"
+book.save
+
+book.title_previously_changed? #=> true
+book.previous_changes
+#=> { "title" => ["Original Title", "The Fifth Season"] }
+```
+
+[source](https://api.rubyonrails.org/classes/ActiveModel/Dirty.html)