Filter by string instead of select in ActiveAdmin

I have belongs_to and has_many assotiations between two models. By default, activeadmin using select input for it. But in my case, it's imposible - because I have a thousands items. When I trying change filter to

filter :item, as: :string

it says that

undefined method `item_id_contains' for #<MetaSearch::Searches::DeliveryTime:0x007fdd57f12db8>

What is the best way to do it right?