Define id_was to get the previous value of the primary key

Currently when we call id_was and we have a custom primary key name
Active Record will return the current value of the primary key. This
make impossible to correctly do an update operation if you change the
id.

Fixes #16413

activerecord/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   Define `id_was` to get the previous value of the primary key.
+
+    Currently when we call id_was and we have a custom primary key name
+    Active Record will return the current value of the primary key. This
+    make impossible to correctly do an update operation if you change the
+    id.
+
+    Fixes #16413.
+
+    *Rafael Mendonça França*
+
 *   Deprecate `DatabaseTasks.load_schema` to act on the current connection.
     Use `.load_schema_current` instead. In the future `load_schema` will
     require the `configuration` to act on as an argument.

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -39,6 +39,12 @@ def id_before_type_cast
         read_attribute_before_type_cast(self.class.primary_key)
       end
 
+      # Returns the primary key previous value.
+      def id_was
+        sync_with_transaction_state
+        attribute_was(self.class.primary_key)
+      end
+
       protected
 
       def attribute_method?(attr_name)
@@ -54,7 +60,7 @@ def define_method_attribute(attr_name)
           end
         end
 
-        ID_ATTRIBUTE_METHODS = %w(id id= id? id_before_type_cast).to_set
+        ID_ATTRIBUTE_METHODS = %w(id id= id? id_before_type_cast id_was).to_set
 
         def dangerous_attribute_method?(method_name)
           super && !ID_ATTRIBUTE_METHODS.include?(method_name)

activerecord/test/cases/primary_keys_test.rb

@@ -5,6 +5,7 @@
 require 'models/movie'
 require 'models/keyboard'
 require 'models/mixed_case_monkey'
+require 'models/dashboard'
 
 class PrimaryKeysTest < ActiveRecord::TestCase
   fixtures :topics, :subscribers, :movies, :mixed_case_monkeys
@@ -164,6 +165,15 @@ def test_auto_detect_primary_key_from_schema
     MixedCaseMonkey.reset_primary_key
     assert_equal "monkeyID", MixedCaseMonkey.primary_key
   end
+
+  def test_primary_key_update_with_custom_key_name
+    dashboard = Dashboard.create!(dashboard_id: '1')
+    dashboard.id = '2'
+    dashboard.save!
+
+    dashboard = Dashboard.first
+    assert_equal '2', dashboard.id
+  end
 end
 
 class PrimaryKeyWithNoConnectionTest < ActiveRecord::TestCase