Fix uniqueness validation with an after_create hook.

record.id_was is nil in after_create/after_save, so we should use
id in these cases.

While this logic feels incomplete, the existing update_record uses the same
logic:
2fda4e0874/activerecord/lib/active_record/relation.rb (L83)

This logic was originally added for a similar problem:
updates not working with after_create hook.

See: 482f8c15b1

Followup to #23581
Fixes #23844

activerecord/lib/active_record/validations/uniqueness.rb

@@ -18,7 +18,7 @@ module ActiveRecord
         relation = build_relation(finder_class, table, attribute, value)
         if record.persisted?
           if finder_class.primary_key
-            relation = relation.where.not(finder_class.primary_key => record.id_was)
+            relation = relation.where.not(finder_class.primary_key => record.id_was || record.id)
           else
             raise UnknownPrimaryKey.new(finder_class, "Can not validate uniqueness for persisted record without primary key.")
           end

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -53,6 +53,14 @@ class CoolTopic < Topic
   validates_uniqueness_of :id
 end
 
+class TopicWithAfterCreate < Topic
+  after_create :set_author
+
+  def set_author
+    update_attributes!(:author_name => "#{title} #{id}")
+  end
+end
+
 class UniquenessValidationTest < ActiveRecord::TestCase
   INT_MAX_VALUE = 2147483647
 
@@ -469,6 +477,16 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert t.save, "Should still save t as unique"
   end
 
+  def test_validate_uniqueness_with_after_create_performing_save
+    TopicWithAfterCreate.validates_uniqueness_of(:title)
+    topic = TopicWithAfterCreate.create!(:title => "Title1")
+    assert topic.author_name.start_with?("Title1")
+
+    topic2 = TopicWithAfterCreate.new(:title => "Title1")
+    refute topic2.valid?
+    assert_equal(["has already been taken"], topic2.errors[:title])
+  end
+
   def test_validate_uniqueness_uuid
     skip unless current_adapter?(:PostgreSQLAdapter)
     item = UuidItem.create!(uuid: SecureRandom.uuid, title: 'item1')