ActiveAdmin meta search filters - filter by association id select dropdown

Question

I have a model setup like this:

• Sale belongs to Product
• Product belongs to Varietal

Now I want to make a select drop down filter for varietal on sales. The only way I've managed to do this is by name.

filter :product_varietal_name,
  as: :select,
  label: 'Varietal name',
  collection: Varietal.names_for_select

But I want to be able to serach by varietal_id since I have an index on that column, it should be much faster. I seem to be unable to do so, however.

filter :product_varietal       # undefined method `product_varietal_eq'
filter :product_varietal_id    # undefined method `product_varietal_eq'
filter :product_varietal_id_eq # undefined method `product_varietal_id_eq_eq'
filter :product_varietal_eq    # undefined method `product_varietal_eq_eq'

There is clearly some magic here around the _id at the end of the filter name. And this magic is not doing what I want.

The only case this does work is when the association is on the Model being viewed. So Sale belongs to Product, and filter :product works as you would expect.

So how can I make a drop down select box filter that searches on sale.product.varietal_id?