gistfile1.rb

class ModelBase < ActiveRecord::Base
end

class BaseA < ModelBase
  has_many :things, :as => :model_a
end

class BaseB < ModelBase
  has_many :things, :as => :model_b
end

class Thing < ActiveRecord::Base
  belongs_to :model_a, :polymorphic => true
  belongs_to :model_b, :polymorphic => true
end

t = Thing.create :model_a => BaseA.first, model_b => BaseB.first
 => #<Thins model_a_type: "ModelBase", :model_b_type, "ModelBase"
                           ^^^^^^^^^
                           Expected BaseA

                                                       ^^^^^^^^^
                                                       Expected BaseB