rails validate in model that value is inside array

ActiveModel::Validations provides a helper method for this. An example call would be:

validates_inclusion_of :type, in: @allowed_types

ActiveRecord::Base is already a ActiveModel::Validations, so there is no need to include anything.

http://apidock.com/rails/ActiveModel/Validations/HelperMethods/validates_inclusion_of

Also, @RadBrad is correct that you should not use type as a column name as it is reserved for STI.

first, change the attribute from type to something else, type is a reserved attrubute name use for Single Table Inheritance and such.

class Thing < ActiveRecord::Base
   validates :mytype, :inclusion=> { :in => @allowed_types }