ActiveModel: Rails’ Framework for Non-Persistent Models

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ActiveModel brings Rails conventions to plain Ruby objects, letting you
use validation, dirty tracking, and form bindings without a database.  
Think of it as ActiveRecord's stripped-down cousin for non-persistent  
models. Key modules include AttributeAssignment (mass assignment),  
Validations (familiar rules like presence checks), Dirty Tracking  
(see what changed), and Conversion (Rails URL helpers).

Hello world.

Example: A UserForm class can include ActiveModel::Model, define  
attributes, add validations, and track changes via changes_applied.  
It works with form_for just like ActiveRecord models but might save  
via API calls instead of a database.

Use cases: form objects, API client wrappers, service objects that  
need validations. Lets you keep Rails patterns consistent even when  
you’re not hitting the database. Not magic – just thoughtful reuse  
of Rails’ internal interfaces. Use modules à la carte or bundle them  
via ActiveModel::Model. Makes your POROs play nice with the Rails  
ecosystem.