Use Dry gems to build a struct which has an active record relation from a specified model as an attribute

active_record_relation_type.rb

require 'dry-struct'
require 'active_record'

module Types
  include Dry.Types

  def self.ArRelation(model_class)
    # Note that ActiveRecord_Relation, ActiveRecord_Associations_CollectionProxy, and ActiveRecord_AssociationRelation
    # are private constants - they may change some day
    Instance(model_class.const_get(:ActiveRecord_Relation)) |
      Instance(model_class.const_get(:ActiveRecord_Associations_CollectionProxy)) |
      Instance(model_class.const_get(:ActiveRecord_AssociationRelation)) |
      Array.of(Instance(model_class))
  end
end

class Foo < ActiveRecord::Base; end
class Bar < ActiveRecord::Base; end

class Thing < Dry::Struct
  attribute :foos, Types.ArRelation(Foo)
end

Thing.new(foos: Foo.none) # => A Thing!
Thing.new(foos: Bar.none) # => Dry::Struct::Error: [Thing.new] #<ActiveRecord::Relation []> (Bar::ActiveRecord_Relation) has invalid type for :foos ...

Great gist! Thanks!