persistent objects

As longevity is primarily a persistence framework, the common currency of the longevity API is the persistent object. Persistent objects are part of your domain model, and they are also the persistence unit - something that you can create, retrieve, update or delete with a longevity repository.

Persistent objects are Scala traits and case classes that meet some basic criteria. Those criteria are laid out in the chapters that follow. The basic idea is that they should form an algebraic data type.

Here’s a simple example:

case class User(
  username: String,
  firstName: String,
  lastName: String)

To tell longevity that this is a persistent object, we mark the class with a @persistent annotation. This annotation requires a type parameter explicating the model:

import longevity.model.annotations.domainModel
import longevity.model.annotations.persistent

@domainModel trait DomainModel

@persistent[DomainModel]
case class User(
  username: String,
  firstName: String,
  lastName: String)

Be sure to declare your persistent class in the same package as, or in a sub-package of, the package you declare your domain model. Assuming you don’t fabricate your own longevity.model.ModelEv[DomainModel], if you declare your persistent class in another package, you will get a compiler error - something about implicit model evidence not being found.

The @persistent annotation expands as follows:

import longevity.model.PType
import longevity.model.annotations.mprops

case class User(
  username: String,
  firstName: String,
  lastName: String)

@mprops object User extends PType[DomainModel, User]

The companion object has been made into a PType[DomainModel, User], which will be used to build the ModelType[DomainModel], which collects all the different parts of your model.

The @mprops macro in turn creates an object props within your companion object, that allows us to reflect over the fields of the User case class. We’ll learn more about how this works in the chapter on properties. The expansion of @mprops looks something like this:

import longevity.model.PType
import longevity.model.ptype.Prop

case class User(
  username: String,
  firstName: String,
  lastName: String)

object User extends PType[DomainModel, User] {
  object props {
    object username extends Prop[User, String]("username")
    object firstName extends Prop[User, String]("firstName")
    object lastName extends Prop[User, String]("lastName")
  }
}