integer_sti.rb

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema[8.1].define(version: 1) do
  create_table :sti_classes, id: :integer do |t|
    t.string :name, null: false
  end

  create_table :examples do |t|
    t.integer :type
    t.timestamps
  end
end

class StiType < ActiveRecord::Type::Value
  def cast(value)
    case value
    when Class       then value.name
    when String, nil then value
    when Integer     then StiClass.name_for(value)
    else
      raise ArgumentError, "Invalid value for StiType: #{value.inspect}"
    end
  end

  alias deserialize cast

  def serialize(value) = StiClass.id_for(value)
end

class StiClass < ActiveRecord::Base
  def self.id_for(name)
    return unless name &&= name.to_s
    @id_for       ||= unscoped.pluck(:name, :id).to_h
    @id_for[name] ||= find_or_create_by!(name: name).id
  rescue ActiveRecord::RecordNotUnique
    retry
  end

  def self.name_for(id)
    @name_for     ||= unscoped.pluck(:id, :name).to_h
    @name_for[id] ||= find(id).name
  rescue ActiveRecord::RecordNotFound
    raise ArgumentError, "Invalid id for StiClass: #{id.inspect}"
  end
end

class Example < ActiveRecord::Base
  attribute :type, StiType.new
end

class SubExample < Example
end

load Gem.bin_path("irb", "irb")

I do realize that this technically isn't single-table inheritance anymore.