need to register subclasses, too

lib/mongoid/traversable.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'mongoid/fields/validators/macro'
+require 'mongoid/model_resolver'
 
 module Mongoid
   # Mixin module included in Mongoid::Document to provide behavior
@@ -32,6 +33,10 @@ def hereditary?
       # rubocop:disable Metrics/AbcSize
       def inherited(subclass)
         super
+
+        # Register the new subclass with the resolver subsystem
+        Mongoid::ModelResolver.register(subclass)
+
         @_type = nil
         subclass.aliased_fields = aliased_fields.dup
         subclass.localized_fields = localized_fields.dup

spec/mongoid/association/referenced/belongs_to/proxy_spec.rb

@@ -2,6 +2,7 @@
 # rubocop:todo all
 
 require "spec_helper"
+require 'support/models/canvas'
 require_relative '../belongs_to_models.rb'
 
 describe Mongoid::Association::Referenced::BelongsTo::Proxy do

spec/mongoid/association/referenced/has_many/buildable_spec.rb

@@ -100,6 +100,10 @@
         Post.where(association.foreign_key => object, 'ratable_type' => 'Rating')
       end
 
+      before do
+        Post.belongs_to :ratable, polymorphic: true
+      end
+
       it "adds the type to the criteria" do
         expect(documents).to eq(criteria)
       end