Use `_relation_for_itself` wherever possible

Anywhere that we're doing `self.class.unscoped.where(primary_key => id)`
is somewhere that someone may want to extend. Even if this method isn't
public API yet, this will make it easier for us to eventually expose an
API around this. Plus, duplicated code makes me sad.

activerecord/lib/active_record/persistence.rb

@@ -332,7 +332,7 @@ def update_columns(attributes)
         verify_readonly_attribute(key.to_s)
       end
 
-      updated_count = self.class.unscoped.where(self.class.primary_key => id).update_all(attributes)
+      updated_count = _relation_for_itself.update_all(attributes)
 
       attributes.each do |k, v|
         write_attribute_without_type_cast(k, v)
@@ -523,8 +523,7 @@ def touch(*names, time: nil)
           changes[column] = write_attribute(column, time)
         end
 
-        primary_key = self.class.primary_key
-        scope = self.class.unscoped.where(primary_key => _read_attribute(primary_key))
+        scope = _relation_for_itself
 
         if locking_enabled?
           locking_column = self.class.locking_column