Demonstrate how to use built-in Active Model and Active Record concepts to store JSON representations of Active Model instances in SQL

serialized_models.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "rails"

  gem "sqlite3", "~> 1.4"
end

require "active_record"
require "minitest/autorun"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

#
# SECTION: ==================== NOT YET ON rails/rails MAIN ====================
#

# DO NOT EDIT UNLESS COPY-PASTING FROM:
# https://github.com/rails/rails/pull/51420
#
module ActiveModel
  Attributes.include(Module.new do
    # Returns a hash of attributes for assignment to the database.
    def attributes_for_database
      @attributes.values_for_database
    end
  end)
end

# DO NOT EDIT UNLESS COPY-PASTING FROM:
# https://github.com/rails/rails/pull/51421
#
module ActiveModel
  module Type
    # Attribute type for a collection of values. It is registered under
    # the +:collection+ key. +:element_type+ option is used to specify elements type
    #
    #   class User
    #     include ActiveModel::Attributes
    #
    #     attribute :lucky_numbers, :collection, element_type: :integer
    #   end
    #
    #   user = User.new(lucky_numbers: [1, 2, 3])
    #   user.lucky_numbers # => [1, 2, 3]
    #
    # Value is wrapped into an Array if not an Array already
    #
    #   User.new(lucky_numbers: 1).lucky_numbers # => [1]
    #
    # Collection elements are coerced by their +:element_type+ type
    #
    #   User.new(lucky_numbers: ["1"]).lucky_numbers # => [1]
    #
    class Collection < Value
      def initialize(**args)
        @element_type = args.delete(:element_type)
        @type_object = Type.lookup(element_type, **args)
        @serializer = args.delete(:serializer) || ActiveSupport::JSON
        super()
      end

      def type
        :collection
      end

      def cast(value)
        return [] if value.nil?
        Array(value).map { |el| @type_object.cast(el) }
      end

      def serializable?(value)
        value.all? { |el| @type_object.serializable?(el) }
      end

      def serialize(value)
        serializer.encode(value.map { |el| @type_object.serialize(el) })
      end

      def deserialize(value)
        serializer.decode(value).map { |el| @type_object.deserialize(el) }
      end

      def assert_valid_value(value)
        return if valid_value?(value)
        raise ArgumentError, "'#{value}' is not a valid #{type} of #{element_type}"
      end

      def changed_in_place?(raw_old_value, new_value)
        old_value = deserialize(raw_old_value)
        return true if old_value.size != new_value.size

        old_value.each_with_index.any? do |raw_old, i|
          @type_object.changed_in_place?(raw_old, new_value[i])
        end
      end

      def valid_value?(value)
        value.is_a?(Array) && value.all? { |el| @type_object.valid_value?(el) }
      end

      private

      attr_reader :element_type, :serializer
    end

    register :collection, Collection
  end
end

# DO NOT EDIT UNLESS COPY-PASTING FROM:
# https://github.com/rails/rails/pull/51420
#
module ActiveModel
  module Type
    class Model < Value # :nodoc:
      def initialize(**args)
        @class_name = args.delete(:class_name)
        @serializer = args.delete(:serializer) || ActiveSupport::JSON
        super
      end

      def changed_in_place?(raw_old_value, value)
        old_value = deserialize(raw_old_value)
        old_value.attributes != value.attributes
      end

      def valid_value?(value)
        return valid_hash?(value) if value.is_a?(Hash)

        value.is_a?(klass)
      end

      def type
        :model
      end

      def serializable?(value)
        value.is_a?(klass)
      end

      def serialize(value)
        serializer.encode(value.attributes_for_database)
      end

      def deserialize(value)
        attributes = serializer.decode(value)
        klass.new(attributes)
      end

      private

      attr_reader :serializer

      def valid_hash?(value)
        # TODO upstream as follow-up to
        # https://github.com/rails/rails/pull/51420#discussion_r1597220609
        attribute_keys = value.keys.map do |key|
          key = key.to_s

          if klass.attribute_alias?(key)
            klass.attribute_alias(key)
          else
            key
          end
        end

        attribute_keys.difference(klass.attribute_names).none?
      end

      def klass
        @_model_type_class ||= @class_name.constantize
      end

      def cast_value(value)
        case value
        when Hash
          klass.new(value)
        else
          klass.new(value.attributes)
        end
      end
    end

    register :model, Model
  end
end

#
# SECTION: =============== Active Model Serialization Extensions ===============
#

module JsonSerializable
  extend ActiveSupport::Concern

  included do
    include ActiveModel::Serializers::JSON

    def assign_attributes(attributes)
      transformed_attributes = attributes.deep_transform_keys do |key|
        deserialize_json_key(key)
      end

      super(transformed_attributes)
    end

    # TODO Upstream as https://github.com/rails/rails/pull/51781
    alias_method :attributes=, :assign_attributes

    def serializable_hash(...)
      super.deep_transform_keys do |key|
        serialize_json_key(key)
      end
    end

    private

    def deserialize_json_key(key) = key

    def serialize_json_key(key) = key
  end

  class_methods do
    def load(json)
      if json.present?
        new.from_json(json)
      end
    end

    def dump(model)
      model.to_json
    end
  end
end

module JsonSerializable::CamelCased
  extend ActiveSupport::Concern

  included do
    include JsonSerializable

    private

    def deserialize_json_key(key) = key.to_s.underscore

    def serialize_json_key(key) = key.camelize(:lower)
  end
end

#
# SECTION: ========================= Application Code ==========================
#

ActiveRecord::Schema.define do
  create_table :webhook_requests, force: true do |t|
    t.text :payload
  end
end

class ApplicationModel
  include ActiveModel::Model
  include ActiveModel::Attributes

  include JsonSerializable
end

class Payload < ApplicationModel
  attribute :request_id, :string
end

class WebhookRequest < ActiveRecord::Base
  serialize :payload, coder: Payload
end

module AnApiService
  class SpecializedData < ApplicationModel
    attribute :value, :integer
  end

  class Payload < ::Payload
    include JsonSerializable::CamelCased

    attribute :data, :collection, default: [], element_type: :model, class_name: "AnApiService::SpecializedData"
  end

  class WebhookRequest < ::WebhookRequest
    serialize :payload, coder: AnApiService::Payload
  end
end

class WebhookRequestTest < ActiveSupport::TestCase
  test "writes payload to JSON, reads to Payload instance" do
    payload = Payload.new(request_id: "abc123")
    webhook_request = WebhookRequest.create!(payload:)

    assert_equal payload.to_json, webhook_request.payload_before_type_cast
    assert_equal payload.request_id, webhook_request.payload.request_id
  end
end

class AnApiService::WebhookRequestTest < ActiveSupport::TestCase
  test "writes to camelCase, reads from snake_case" do
    payload = AnApiService::Payload.new(request_id: "abc123")
    webhook_request = AnApiService::WebhookRequest.create!(payload:)

    assert_equal({requestId: "abc123", data: []}.to_json, webhook_request.payload_before_type_cast)
    assert_equal payload.request_id, webhook_request.payload.request_id
  end

  test "supports nesting collections of objects" do
    payload = AnApiService::Payload.new(requestId: nil, data: [{value: "1"}, {value: "2"}])
    webhook_request = AnApiService::WebhookRequest.create!(payload:)

    assert_equal({requestId: nil, data: [{value: 1}, {value: 2}]}.to_json, webhook_request.payload_before_type_cast)
    assert_equal 1, webhook_request.payload.data.first.value
    assert_equal 2, webhook_request.payload.data.second.value
  end
end

One more thing! I think you're looking for stopdoc and startdoc in case nodoc doesn't work. You can also define the two attributes in one go:

# :stopdoc:
class_attribute :_from_json_formatter, :_to_json_formatter, instance_writer: false, default: KeyFormatter::Identity.new
# :startdoc:

You could also rename format_keys! to call and then set default: :itself.to_proc above, if you want to spare the Identity class. I suppose you could also leave it blank and do formatter&.call(hash) || hash though having the simplified call site is nice.

Played around with the as_json definition a little more, don't think I've ever realized how pattern matching could provide a fallback, sorta like:

def as_json(options = nil)
  hash = serializable_hash(options).then { _to_json_formatter.call _1 }.as_json

  options in { root: } or root = include_root_in_json
  if root
    root = model_name.element if root == true
    { root => hash }
  else
    hash
  end
end

Also just found a recent PR comment that talks more broadly about the expectations of as_json rails/rails#52448 (comment)

Ok, one more thought. Since there's also ActiveRecord::Base.include_root_in_json and as_json(root: true) and from_json(json, true) (no keyword argument there!), I wonder if we should allow assigning that too?

json_formatting include_root: true, to: :underscore, from: -> { _1.camelize :lower }

json_schema include_root: true, deserialize_keys: :underscore, serialize_keys: -> { _1.camelize :lower }, include: :posts # Could maybe even support default arguments to `as_json`?