allow setting of a demodulized class name when using STI

If your STI class looks like this:

```ruby
class Company < ActiveRecord::Base
  self.store_full_sti_class = false

  class GoodCo < Company
  end

  class BadCo < Company
  end
end
```

The expectation (which is valid) is that the `type` in the database is saved as
`GoodCo` or `BadCo`. However, another expectation should be that setting `type`
to `GoodCo` would correctly instantiate the object as a `Company::GoodCo`. That
second expectation is what this should fix.

1 files changed, 8 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 24098f72dc..a7801963e8 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -198,14 +198,16 @@ module ActiveRecord
       def subclass_from_attributes(attrs)
         subclass_name = attrs.with_indifferent_access[inheritance_column]
 
-        if subclass_name.present? && subclass_name != self.name
-          subclass = subclass_name.safe_constantize
+        if subclass_name.present?
+          subclass = find_sti_class(subclass_name)
 
-          unless descendants.include?(subclass)
-            raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
-          end
+          if subclass.name != self.name
+            unless descendants.include?(subclass)
+              raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
+            end
 
-          subclass
+            subclass
+          end
         end
       end
     end