2

I think the issue is that you’re lacking the polymorphic hints on the associations. The following works for me:

module Product
  class Base
    include Mongoid::Document

    embeds_many :variants, class_name: 'Variant::Base', as: :product
  end

  class Custom < Base
  end
end

module Variant
  class Base
    include Mongoid::Document

    embedded_in :product, polymorphic: true

    field :type_id, type: String
  end

  class Custom < Base
  end
end

product = Product::Custom.new
variant = product.variants.find_or_initialize_by(type_id: 'h', _type: 'Variant::Custom')

p product
p variant

product.save

product = Product::Base.find(product._id)
p product
p product.variants

The key is the as: :product option on embeds_many , and the polymorphic: true option on the embedded_in. Does this work for you?