ActiveModel is a gem that provides common interfaces and functionality for model classes, including validation, dirty tracking, callbacks, and serialization. It includes modules like ActiveModel::Validations, ActiveModel::Dirty, and ActiveModel::Callbacks that define APIs for validations, detecting changes, and hooks. While part of Rails, ActiveModel can also be used independently with the 'activemodel' gem. It provides fundamental modeling frameworks that both ActiveRecord and other ORM/ODM gems are built upon.

1. ActiveModel
ActiveRecord’s useful stuff

2. Gem Spec
• Part of Rails gem package
• Own standalone gem:
• gem ‘activemodel’
• Dependencies:
• gem ‘activesupport’

3. ActiveModel Packages
• ActiveModel::AttributeMethods
• ActiveModel::Callbacks
• ActiveModel::Conversion
• ActiveModel::Dirty
• ActiveModel::Naming
• ActiveModel::Observer
• ActiveModel::Serialization
• ActiveModel::Translation
• ActiveModel::Validations

4. ActiveModel Packages
• ActiveModel::AttributeMethods
• ActiveModel::Callbacks
• ActiveModel::Conversion
• ActiveModel::Dirty
• ActiveModel::Naming
• ActiveModel::Observer
• ActiveModel::Serialization
• ActiveModel::Translation
• ActiveModel::Validations

5. Validations
• Most Rails validations originate from ActiveModel
• Including ActiveModel::Validations gives you
• #valid?
• #errors
• Class methods for validation API

6. Validations API
declarative
• validates_acceptance_of • validates_exclusion_of
• validates_confirmation_of • validates_length_of
• validates_each • validates_numericality_of
• validates_format_of • validates_presence_of
• validates_inclusion_of • validates_with

7. Validations API
short form
• validates :field_name, :validator => options

8. Validation API
• validates_each :field_name, &block
• validate :method_name
• validate &block
• validates_with ValidatorClass

9. Callbacks
• In ActiveRecord, this is how before_create,
after_create and around_create are made.
• Two different steps: register callbacks and
execute callbacks.

10. Observers
• Use observers to decouple logic from
models.
• Observers use convention to identify
targets: ModelObserver observes Model
unless explicitly overridden.

11. Serialization
• Allows for models to define how they
would like to be serialized in a variety of
formats (including XML and JSON).
• Important for creation of presenters that
get sent as JSON in RESTful API.

12. Dirty
• ActiveModel provides an API for detecting
whether an object is “dirty” or not
(changed).
• Could be useful for operations that may or
may not be idempotent and you need to
know which (e.g. marking a message as
read).

13. Questions?