'cause I want to receive one or the other and actually coerce the value

collection_contract.rb

module CollectionContract
  extend ActiveSupport::Concern
  module ClassMethods
    
    #I want to be able to say what I expect and do the appropriate casting if needed
    #in my specific use-case, these are the only two possible arg types
    def collection_method(name, opts = nil, &definition)
      opts ||= {}
      define_method name do |*args|
        rules = {
          Array => ->(x){x.to_a},
          ActiveRecord::Relation => ->(x){opts[:of].where(id: x.map(&:id))}
        }
        transformed = args.map do |arg|
          rules.reduce(arg) do |last_transformation, current_rule|
             expected_type, transformation = current_rule
             if !arg.is_a?(expected_type) && opts[:expects] = expected_type
               transformation[last_transformation]
             else
               last_transformation
             end
          end
        end
        definition[*transformed]
      end
    end
  end
end

example.rb

class DoesStuffWithCollections
  include CollectionContract

   collection_method :extremes, expects: ActiveRecord::Relation, of: User do |users|
     users.minmax_by(&:popularity).map(&:username)
   end
end

get_array_of_user_ids_from_somewhere = ->(){:magic}

DoesStuffWithCollections.new.extremes(get_array_of_user_ids_from_somewhere)