Only Option Not Correct for Nested Value

I have a couple entities that look like this:

class Foo::Entity < Grape::Entity
  expose :id, :foo_name
end

class Bar::Entity < Grape::Entity
  expose :id, :stuff, :things

  expose :foo, :using => Foo::Entity
end

In the resource for Bar, I'm doing

get '/' do
  present @bars.index(:limit => @limit), :only => @only
end

For the first layer, this works great. For example, passing [:id, :things] to @only works perfectly. Passing nothing to @only results in me getting the full Foo object too, which is exactly what I want.

The issue arises when I pass [:id, :things, {:foo => [:foo_name]}]. Instead of the expected id, things, and foo hash containing foo_name, I get back id, things and a foo hash containing just the foo object's id.

Start by reproducing the nested Foo::Entity exposure through the Bar resource's present @bars.index(:limit => @limit), :only => @only call. Compare the result for [:id, :things, {:foo => [:foo_name]}] with the expected nested foo_name output, and consider the issue complete when the nested selection returns that field rather than only id.