This should do the trick.  
```ruby 
+  child :locations => :locations
```
I tried tries, but it causes an error and apparently :object_root => false isn't needed like I thought.
```ruby 
  child :locations => :locations, :object_root => false
```
Somebody else also reported this issue from 0.9 to 0.9 on STI models https://github.com/nesquena/rabl/issues/505