ActiveAdmin Group Select With Formstatic

option_groups_from_collection_for_select.rb

class Category < ApplicationRecord
  has_many :types
end

class Type < ApplicationRecord
  belongs_to :category
end

# Admin DSL. you need to pass the the selected type_id by passing the form object value.
f.input :category
f.input :type, as: :select, collection: option_groups_from_collection_for_select(Category.all, :types, :name, :id, :name, f.object.type_id)