Agile Web Development with Rails 5, 1st Edition

Models, Views, and Controllers

Back in 1979, Trygve Reenskaug came up with a new architecture for developing interactive applications. In his design, applications were broken into three types of components: models, views, and controllers.

The model is responsible for maintaining the state of the application. Sometimes this state is transient, lasting for just a couple of interactions with the user. Sometimes the state is permanent and is stored outside the application, often in a database.

A model is more than data; it enforces all the business rules that apply to that data. For example, if a discount shouldn’t be applied to orders of less than $20, the model enforces the constraint. This makes sense; by putting the implementation of these ...