Shard Detail

active-model v4.2.3

A rails-esque model framework for crystal lang

Install & Use

Add the following code to your project's shard.yml under:

dependencies to use in production
- OR -
development_dependencies to use in development

  github: spider-gazelle/active-model


Spider-Gazelle ActiveModel

CI Crystal Version

Active Model provides a known set of interfaces for usage in model classes. Active Model also helps with building custom ORMs.


Please also checkout the detailed guide.

Active Model

ActiveModel::Model should be used as the base class for your ORM

require "active-model"

class Person < ActiveModel::Model
  attribute name : String = "default value"
  attribute age : Int32

p = Person.from_json("\"name\": \"Bob Jane\"") # => "Bob Jane"
p.to_json # => "\"name\":\"Bob Jane\""
p.attributes # => {:name => "Bob Jane", :age => nil}

p.age = 32
p.attributes # => {:name => "Bob Jane", :age => 32}

The attribute macro takes two parameters. The field name with type and an optional default value.

You can also define enum attributes!
The default serialisation for enums is to a downcased string. Use Enum::ValueConverter(T) if you want to serialise to the value backing members of the enum.

require "active-model"

class Order < ActiveModel::Model
  enum Product

  enum Size

  attribute product : Product = Product::Fries
  attribute size : Size = Size::ExtraMedium, converter: Enum::ValueConverter(Size)


ActiveModel::Validators is a mix-in that you include in your class. Similar to those supported by Rails:

require "active-model"

class Person < ActiveModel::Model
  include ActiveModel::Validation

  attribute name : String
  attribute age : Int32

  validates :name, presence: true, length: { minimum: 3 }
  validates :age, presence: true, numericality: {greater_than: 5}

The validate macro takes three parameters. The symbol of the field and the message that will display when the validation fails. The third is a Proc that is provided an instance of self and returns either true or false.

To check to see if your instance is valid, call valid?. Each Proc will be called and if any of them fails, an errors Array with the messages is returned.

If no Symbol is provided as a first parameter, the errors will be added to the :base field.

person = "JD")
person.valid?.should eq false
person.errors[0].to_s.should eq "Name is too short"

Dirty Checking

Changes to attributes are tracked throughout the lifetime of the model. Similar to Rails:

person = "JD")
person.changed? # => true
person.changed_attributes # => {:name => "JD"}
person.name_changed? # => true
person.name_change # => {nil, "JD"}
person.name_was # => nil

person.changed? # => false


Register before/after callbacks for create, update, delete, save methods. You must define the method you wish to register callbacks for.
Registered callbacks are invoked through wrapping crud logic with the run_create_callbacks, run_update_callbacks, etc. functions

require "active-model"

class Person < ActiveModel::Model
  include ActiveModel::Callbacks

  attribute name : String
  attribute age : Int32

  before_save :capitalize

  def capitalize
    @name = @name.capitalize

  def save
    run_save_callbacks do
      # save to database


The serialization_group argument to attribute accepts an Array(Symbol) or Symbol. This will include the attribute in a generated serializer, #to_<group>_json.

The define_to_json macro allows for defining subset serializations via only and except arguments. The methods argument allows for inclusion of instance methods in the serializer.

require "active-model"

class SerializationGroups < ActiveModel::Model
  attribute everywhere : String = "hi", serialization_group: [:admin, :user, :public]
  attribute joined : Int64 = 0, serialization_group: [:admin, :user]
  attribute mates : Int64 = 0, serialization_group: :user
  attribute another : String = "ok"

  define_to_json :some, only: [:joined, :another]
  define_to_json :most, except: :everywhere
  define_to_json :method, only: :joined, methods: :foo

  getter foo = "foo"

m =
m.to_public_json # {"everywhere":"hi"}
m.to_admin_json  # {"everywhere":"hi","joined":0}
m.to_user_json   # {"everywhere":"hi","joined":0,"mates":1}
m.to_some_json   # {"joined":0,"another":"ok"}
m.to_most_json   # {"joined":0,"mates":0,"another":"ok"}
m.to_method_json # {"joined":0,"foo":"foo"}