Add deprecation warning to `ActiveRecord::Relation#update`

When passing an instance of `ActiveRecord::Base` to `#update`, it would
internally call `#find`, resulting in a misleading deprecation warning.

This change gives this deprecated use of `#update` its own, meaningful
warning.

1 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 8256762f96..7521f0573a 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1541,6 +1541,13 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal 'David', topic2.reload.author_name
   end
 
+  def test_update_on_relation_passing_active_record_object_is_deprecated
+    topic = Topic.create!(title: 'Foo', author_name: nil)
+    assert_deprecated(/update/) do
+      Topic.where(id: topic.id).update(topic, title: 'Bar')
+    end
+  end
+
   def test_distinct
     tag1 = Tag.create(:name => 'Foo')
     tag2 = Tag.create(:name => 'Foo')