Castkit

Castkit is a lightweight, type-safe data object system for Ruby. It provides a declarative DSL for defining data transfer objects (DTOs) with built-in support for typecasting, validation, nested data structures, serialization, deserialization, and contract-driven programming.

Inspired by tools like Jackson (Java) and Python dataclasses, Castkit brings structured data modeling to Ruby in a way that emphasizes:

  • Simplicity: Minimal API surface and predictable behavior.
  • Explicitness: Every field and type is declared clearly.
  • Composition: Support for nested objects, collections, and modular design.
  • Performance: Fast and efficient with minimal runtime overhead.
  • Extensibility: Easy to extend with custom types, serializers, and integrations.

Castkit is designed to work seamlessly in service-oriented and API-driven architectures, providing structure without overreach.

πŸš€ Features

Configuration

Castkit provides a global configuration interface to customize behavior across the entire system. You can configure Castkit by passing a block to Castkit.configure.

Castkit.configure do |config|
  config.enable_warnings = false
  config.enforce_typing = true
end

βš™οΈ Available Settings

Option Type Default Description
enable_warnings Boolean true Enables runtime warnings for misconfigurations.
enforce_typing Boolean true Raises if type mismatch during load (e.g., true vs. "true").
enforce_attribute_access Boolean true Raises if an unknown access level is defined.
enforce_unwrapped_prefix Boolean true Requires unwrapped: true when using attribute prefixes.
enforce_array_options Boolean true Raises if an array attribute is missing the of: option.
raise_type_errors Boolean true Raises if an unregistered or invalid type is used.
strict_by_default Boolean true Applies strict: true by default to all DTOs and Contracts.

πŸ”§ Type System

Castkit comes with built-in support for primitive types and allows registration of custom ones:

Default types

{
        array: Castkit::Types::Collection,
        boolean: Castkit::Types::Boolean,
        date: Castkit::Types::Date,
        datetime: Castkit::Types::DateTime,
        float: Castkit::Types::Float,
        hash: Castkit::Types::Base,
        integer: Castkit::Types::Integer,
        string: Castkit::Types::String
}

Type Aliases

Alias Canonical
collection array
bool boolean
int integer
map hash
number float
str string
timestamp datetime
uuid string

Registering Custom Types

Castkit.configure do |config|
  config.register_type(:mytype, MyTypeClass, aliases: [:custom])
end

Attribute DSL

Castkit attributes define the shape, type, and behavior of fields on a DataObject. Attributes are declared using the attribute method or shorthand type methods provided by Castkit::Core::AttributeTypes.

class UserDto < Castkit::DataObject
  string :name, required: true
  boolean :admin, default: false
  array :tags, of: :string, ignore_nil: true
end

🧠 Supported Types

Castkit supports a strict set of primitive types defined in Castkit::Configuration::DEFAULT_TYPES and aliased in TYPE_ALIASES.

Canonical Types:

  • :array
  • :boolean
  • :date
  • :datetime
  • :float
  • :hash
  • :integer
  • :string

Type Aliases:

Castkit provides shorthand aliases for common primitive types:

Alias Canonical Description
collection array Alias for arrays
bool boolean Alias for true/false types
int integer Alias for integer values
map hash Alias for hashes (key-value pairs)
number float Alias for numeric values
str string Alias for strings
timestamp datetime Alias for date-time values
uuid string Commonly used for identifiers

No other types are supported unless explicitly registered via Castkit.configuration.register_type.

βš™οΈ Attribute Options

Option Type Default Description
required Boolean true Whether the field is required on initialization.
default Object/Proc nil Default value or lambda called at runtime.
access Array [:read, :write] Controls read/write visibility.
ignore_nil Boolean false Exclude nil values from serialization.
ignore_blank Boolean false Exclude empty strings, arrays, and hashes.
ignore Boolean false Fully ignore the field (no serialization/deserialization).
composite Boolean false Used for computed, virtual fields.
transient Boolean false Excluded from serialized output.
unwrapped Boolean false Merges nested DataObject fields into parent.
prefix String nil Used with unwrapped to prefix keys.
aliases Array [] Accept alternative keys during deserialization.
of: Symbol nil Required for :array attributes.
validator: Proc nil Optional callable that validates the value.

πŸ”’ Access Control

Access determines when the field is considered readable/writable.

string :email, access: [:read]
string :password, access: [:write]

🧩 Attribute Grouping

Castkit supports grouping attributes using required and optional blocks to reduce repetition and improve clarity when defining large DTOs.

Example

class UserDto < Castkit::DataObject
  required do
    string :id
    string :name
  end

  optional do
    integer :age
    boolean :admin
  end
end

This is equivalent to:

class UserDto < Castkit::DataObject
  string :id # required: true
  string :name # required: true
  integer :age, required: false
  boolean :admin, required: false
end

Grouped declarations are especially useful when your DTO has many optional fields or a mix of required/optional fields across different types.

🧬 Unwrapped & Composite

class Metadata < Castkit::DataObject
  string :locale
end

class PageDto < Castkit::DataObject
  dataobject :metadata, unwrapped: true, prefix: "meta"
end

# Serializes as:
# { "meta_locale": "en" }

Composite Attributes

Composite fields are computed virtual attributes:

class ProductDto < Castkit::DataObject
  string :name, required: true
  string :sku, access: [:read]
  float :price, default: 0.0

  composite :description, :string do
    "#{name}: #{sku} - #{price}"
  end
end

πŸ” Transient Attributes

Transient fields are excluded from serialization and can be defined in two ways:

class ProductDto < Castkit::DataObject
  string :id, transient: true

  transient do
    string :internal_token
  end
end

πŸͺž Aliases and Key Paths

string :email, aliases: ["emailAddress", "user.email"]

dto.load({ "emailAddress" => "[email protected]" })

πŸ§ͺ Example

class ProductDto < Castkit::DataObject
  string :name, required: true
  float :price, default: 0.0, validator: ->(v) { raise "too low" if v < 0 }
  array :tags, of: :string, ignore_blank: true
  string :sku, access: [:read]

  composite :description, :string do
    "#{name}: #{sku} - #{price}"
  end

  transient do
    string :id
  end
end

DataObjects

Castkit::DataObject is the base class for all structured DTOs. It offers a complete lifecycle for data ingestion, transformation, and output, supporting strict typing, validation, access control, aliasing, serialization, and root-wrapped payloads.

✍️ Defining a DTO

class UserDto < Castkit::DataObject
  string :id
  string :name
  integer :age, required: false
end

πŸš€ Instantiation & Usage

user = UserDto.new(name: "Alice", age: 30)
user.to_h       #=> { name: "Alice", age: 30 }
user.to_json    #=> '{"name":"Alice","age":30}'

βš–οΈ Strict Mode vs. Unknown Key Handling

By default, Castkit operates in strict mode and raises if unknown keys are passed. You can override this:

class LooseDto < Castkit::DataObject
  strict false
  ignore_unknown true      # equivalent to strict false
  warn_on_unknown true     # emits a warning instead of raising
end

To build a relaxed version dynamically:

LooseClone = MyDto.relaxed(warn_on_unknown: true)

🧱 Root Wrapping

class WrappedDto < Castkit::DataObject
  root :user
  string :name
end

WrappedDto.new(name: "Test").to_h
#=> { "user" => { "name" => "Test" } }

πŸ“¦ Deserialization Helpers

You can deserialize using:

UserDto.from_h(hash)
UserDto.deserialize(hash)

πŸ” Conversion from/to Contract

contract = UserDto.to_contract
UserDto.validate!(id: "123", name: "Alice")

from_contract = Castkit::DataObject.from_contract(contract)

πŸ”„ Serializer Override

To override default serialization behavior:


class CustomSerializer < Castkit::Serializers::Base
  def call
    { payload: object.to_h }
  end
end

class MyDto < Castkit::DataObject
  string :field
  serializer CustomSerializer
end

πŸ” Tracking Unknown Fields

dto = UserDto.new(name: "Alice", foo: "bar")
dto.unknown_attributes
#=> { foo: "bar" }

πŸ“€ Registering a Contract

UserDto.register!(as: :User)
# Registers under Castkit::DataObjects::User

Contracts

Castkit::Contract provides a lightweight mechanism for validating structured input without requiring a full data model. Ideal for validating service inputs, API payloads, or command parameters.

πŸ›  Defining Contracts

You can define a contract using the .build DSL:

UserContract = Castkit::Contract.build(:user) do
  string :id
  string :email, required: false
end

Or subclass directly:


class MyContract < Castkit::Contract::Base
  string :id
  integer :count, required: false
end

πŸ§ͺ Validation

UserContract.validate(id: "123")
UserContract.validate!(id: "123")

Returns a Castkit::Contract::Result with:

  • #success? / #failure?
  • #errors hash
  • #to_h / #to_s

βš–οΈ Strict, Loose, and Warn Modes

LooseContract = Castkit::Contract.build(:loose, strict: false) do
  string :token
end

StrictContract = Castkit::Contract.build(:strict, allow_unknown: false, warn_on_unknown: true) do
  string :id
end

πŸ”„ Converting From DataObject

class UserDto < Castkit::DataObject
  string :id
  string :email
end

UserContract = Castkit::Contract.from_dataobject(UserDto)

↔️ Converting Back to DTO

UserDto = UserContract.to_dataobject
# or
UserDto = UserContract.dataobject

πŸ“€ Registering a Contract

UserContract.register!(as: :UserInput)
# Registers under Castkit::Contracts::UserInput

🧱 Supported Options in Contract Attributes

Only a subset of options are supported:

  • required
  • aliases
  • min, max, format
  • of (for arrays)
  • validator
  • unwrapped, prefix
  • force_type

🧩 Validating Nested DTOs

class AddressDto < Castkit::DataObject
  string :city
end

class UserDto < Castkit::DataObject
  string :id
  dataobject :address, of: AddressDto
end

UserContract = Castkit::Contract.from_dataobject(UserDto)
UserContract.validate!(id: "abc", address: { city: "Boston" })

Advanced Usage (coming soon)

Castkit is designed to be modular and extendable. Future guides will cover:

  • Custom serializers (Castkit::Serializers::Base)
  • Integration layers:
    • castkit-activerecord for syncing with ActiveRecord models
    • castkit-msgpack for binary encoding
    • castkit-oj for high-performance JSON
  • OpenAPI-compatible schema generation
  • Declarative enums and union type helpers
  • Circular reference detection in nested serialization

Plugins

Castkit supports modular extensions through a lightweight plugin system. Plugins can modify or extend the behavior of Castkit::DataObject classes, such as adding serialization support, transformation helpers, or framework integrations.

Plugins are just Ruby modules and can be registered and activated globally or per-class.

πŸ“¦ Activating Plugins

Plugins can be activated on any DataObject or at runtime:

module MyPlugin
  def self.setup!(klass)
    # Optional: called after inclusion
    klass.string :plugin_id
  end

  def plugin_feature
    "Enabled!"
  end
end

Castkit.configure do |config|
  config.register_plugin(:my_plugin, MyPlugin)
end

class MyDto < Castkit::DataObject
  Castkit::Plugins.activate(self, :my_plugin)
end

This includes the MyPlugin module into MyDto and calls MyPlugin.setup!(MyDto) if defined.

🧩 Registering Plugins

Plugins must be registered before use:

Castkit.configure do |config|
  config.register_plugin(:oj, Castkit::Plugins::Oj)
end

You can then activate them:

Castkit::Plugins.activate(MyDto, :oj)

🧰 Plugin API

Method Description
Castkit::Plugins.register(:name, mod) Registers a plugin under a custom name.
Castkit::Plugins.activate(klass, *names) Includes one or more plugins into a class.
Castkit::Plugins.lookup!(:name) Looks up the plugin by name or constant.

πŸ“ Plugin Structure

Castkit looks for plugins under the Castkit::Plugins namespace by default:

module Castkit
  module Plugins
    module Oj
      def self.setup!(klass)
        klass.include SerializationSupport
      end
    end
  end
end

To activate this:

Castkit::Plugins.activate(MyDto, :oj)

You can also manually register plugins not under this namespace.

βœ… Example Use Case

module Castkit
  module Plugins
    module Timestamps
      def self.setup!(klass)
        klass.datetime :created_at
        klass.datetime :updated_at
      end
    end
  end
end

Castkit::Plugins.activate(UserDto, :timestamps)

This approach allows reusable, modular feature sets across DTOs with clean setup behavior.

Castkit CLI

Castkit includes a command-line interface to help scaffold and inspect DTO components with ease.

The CLI is structured around two primary commands:

  • castkit generate β€” scaffolds boilerplate for Castkit components.
  • castkit list β€” introspects and displays registered or defined components.

✨ Generate Commands

The castkit generate command provides subcommands for creating files for all core Castkit component types.

🧱 DataObject

castkit generate dataobject User name:string age:integer

Creates:

  • lib/castkit/data_objects/user.rb
  • spec/castkit/data_objects/user_spec.rb

πŸ“„ Contract

castkit generate contract UserInput id:string email:string

Creates:

  • lib/castkit/contracts/user_input.rb
  • spec/castkit/contracts/user_input_spec.rb

πŸ”Œ Plugin

castkit generate plugin Oj

Creates:

  • lib/castkit/plugins/oj.rb
  • spec/castkit/plugins/oj_spec.rb

πŸ§ͺ Validator

castkit generate validator Money

Creates:

  • lib/castkit/validators/money.rb
  • spec/castkit/validators/money_spec.rb

🧬 Type

castkit generate type money

Creates:

  • lib/castkit/types/money.rb
  • spec/castkit/types/money_spec.rb

πŸ“¦ Serializer

castkit generate serializer Json

Creates:

  • lib/castkit/serializers/json.rb
  • spec/castkit/serializers/json_spec.rb

You can disable test generation with --no-spec.

πŸ“‹ List Commands

The castkit list command provides an interface to view internal Castkit definitions or project-registered components.

🧾 List Types

castkit list types

Displays a grouped list of:

  • Native types (defined by Castkit)
  • Custom types (registered via Castkit.configure)

Example:

Native Types:
  Castkit::Types::String         - :string, :str, :uuid

Custom Types:
  MyApp::Types::Money            - :money

πŸ” List Validators

castkit list validators

Displays all validator classes defined in lib/castkit/validators or custom-defined under Castkit::Validators.

Castkit validators are tagged [Castkit], and others as [Custom].

πŸ“‘ List Contracts

castkit list contracts

Lists all contracts in the Castkit::Contracts namespace and related files.

πŸ“¦ List DataObjects

castkit list dataobjects

Lists all DTOs in the Castkit::DataObjects namespace.

πŸ§ͺ List Serializers

castkit list serializers

Lists all serializer classes and their source origin.

🧰 Example Usage

castkit generate dataobject Product name:string price:float
castkit generate contract ProductInput name:string

castkit list types
castkit list validators

The CLI is designed to provide a familiar Rails-like generator experience, tailored for Castkit’s data-first architecture.

Testing

You can test DTOs and Contracts by treating them like plain Ruby objects:

dto = MyDto.new(name: "Alice")
expect(dto.name).to eq("Alice")

You can also assert validation errors:

expect {
  MyDto.new(name: nil)
}.to raise_error(Castkit::AttributeError, /name is required/)

Compatibility

  • Ruby 2.7+
  • Zero dependencies (uses core Ruby)

License

MIT. See LICENSE.

πŸ™ Credits

Created with ❀️ by Nathan Lucas