From a0bb19fbfa590a3a773a24bc018b92fd5ba93b4d Mon Sep 17 00:00:00 2001
From: David Heinemeier Hansson <david@loudthinking.com>
Date: Thu, 1 Aug 2019 15:38:03 -0700
Subject: [PATCH] Add *_previously_was attribute methods when dirty tracking
 (#36836)

---
 activemodel/CHANGELOG.md              | 6 ++++++
 activemodel/lib/active_model/dirty.rb | 8 +++++++-
 activerecord/test/cases/dirty_test.rb | 6 ++++++
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 9d77564c61..c301f4766d 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Add *_previously_was attribute methods when dirty tracking. Example:
 
+        pirate.update(catchphrase: "Ahoy!")
+        pirate.previous_changes["catchphrase"] # => ["Thar She Blows!", "Ahoy!"]
+        pirate.catchphrase_previously_was # => "Thar She Blows!"
+
+    *DHH*
 
 Please check [6-0-stable](https://github.com/rails/rails/blob/6-0-stable/activemodel/CHANGELOG.md) for previous changes.
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index aaefe00c83..245616502d 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -84,6 +84,7 @@ module ActiveModel
   #   person.previous_changes         # => {"name" => [nil, "Bill"]}
   #   person.name_previously_changed? # => true
   #   person.name_previous_change     # => [nil, "Bill"]
+  #   person.name_previously_was      # => nil
   #   person.reload!
   #   person.previous_changes         # => {}
   #
@@ -122,7 +123,7 @@ module ActiveModel
 
     included do
       attribute_method_suffix "_changed?", "_change", "_will_change!", "_was"
-      attribute_method_suffix "_previously_changed?", "_previous_change"
+      attribute_method_suffix "_previously_changed?", "_previous_change", "_previously_was"
       attribute_method_affix prefix: "restore_", suffix: "!"
     end
 
@@ -180,6 +181,11 @@ module ActiveModel
       mutations_before_last_save.changed?(attr_name.to_s)
     end
 
+    # Dispatch target for <tt>*_previously_was</tt> attribute methods.
+    def attribute_previously_was(attr_name) # :nodoc:
+      mutations_before_last_save.original_value(attr_name.to_s)
+    end
+
     # Restore all previous data of the provided attributes.
     def restore_attributes(attr_names = changed)
       attr_names.each { |attr_name| restore_attribute!(attr_name) }
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index a2a501a794..6aca7d93ad 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -491,6 +491,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 4, pirate.previous_changes.size
     assert_equal [nil, "arrr"], pirate.previous_changes["catchphrase"]
+    assert_equal nil, pirate.catchphrase_previously_was
     assert_equal [nil, pirate.id], pirate.previous_changes["id"]
     assert_nil pirate.previous_changes["updated_on"][0]
     assert_not_nil pirate.previous_changes["updated_on"][1]
@@ -507,6 +508,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 4, pirate.previous_changes.size
     assert_equal [nil, "arrr"], pirate.previous_changes["catchphrase"]
+    assert_equal nil, pirate.catchphrase_previously_was
     assert_equal [nil, pirate.id], pirate.previous_changes["id"]
     assert_includes pirate.previous_changes, "updated_on"
     assert_includes pirate.previous_changes, "created_on"
@@ -525,6 +527,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 2, pirate.previous_changes.size
     assert_equal ["arrr", "Me Maties!"], pirate.previous_changes["catchphrase"]
+    assert_equal "arrr", pirate.catchphrase_previously_was
     assert_not_nil pirate.previous_changes["updated_on"][0]
     assert_not_nil pirate.previous_changes["updated_on"][1]
     assert_not pirate.previous_changes.key?("parrot_id")
@@ -539,6 +542,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 2, pirate.previous_changes.size
     assert_equal ["Me Maties!", "Thar She Blows!"], pirate.previous_changes["catchphrase"]
+    assert_equal "Me Maties!", pirate.catchphrase_previously_was
     assert_not_nil pirate.previous_changes["updated_on"][0]
     assert_not_nil pirate.previous_changes["updated_on"][1]
     assert_not pirate.previous_changes.key?("parrot_id")
@@ -551,6 +555,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 2, pirate.previous_changes.size
     assert_equal ["Thar She Blows!", "Ahoy!"], pirate.previous_changes["catchphrase"]
+    assert_equal "Thar She Blows!", pirate.catchphrase_previously_was
     assert_not_nil pirate.previous_changes["updated_on"][0]
     assert_not_nil pirate.previous_changes["updated_on"][1]
     assert_not pirate.previous_changes.key?("parrot_id")
@@ -563,6 +568,7 @@ class DirtyTest < ActiveRecord::TestCase
 
     assert_equal 2, pirate.previous_changes.size
     assert_equal ["Ahoy!", "Ninjas suck!"], pirate.previous_changes["catchphrase"]
+    assert_equal "Ahoy!", pirate.catchphrase_previously_was
     assert_not_nil pirate.previous_changes["updated_on"][0]
     assert_not_nil pirate.previous_changes["updated_on"][1]
     assert_not pirate.previous_changes.key?("parrot_id")