Add Inspect Previous Changes To ActiveRecord Object as a rails til

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

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+# 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)