This can be used so that when building records via nested attributes the correct subclass is used when creating the record.

sti_nested_attributes_association_builder.rb

ActiveRecord::Reflection::AbstractReflection.class_eval do
  def build_association(*options, &block)
    attributes = if options.first.is_a?(Hash)
      options.first.with_indifferent_access
    end

    if attributes && attributes[:type].present?
      attributes[:type].constantize.new(*options, &block)
    else
      klass.new(*options, &block)
    end
  end
end