keys

Keys are composed of a single property whose type is key value for the persistent object. To declare a key, we use PType method key like so:

import longevity.model.annotations.keyVal
import longevity.model.annotations.persistent

@keyVal[User]
case class Username(username: String)

@persistent
case class User(
  username: Username,
  firstName: String,
  lastName: String)

object User {
  implicit val usernameKey = key(props.username)
}

We make the key implicit because it is used as an implicit parameter to the Repo.retrieve and Repo.retrieveOne methods, as we will see later.

Keys play two important roles in your domain model. First, they indicate that a key value should uniquely identify a persistent object. So in the above example, no two users can have the same username. Second, they indicate that looking up a persistent object by key value should be fast.

We can declare multiple keys, and composite keys, just as easily. Here, for instance, we add an ill-advised composite key on a firstName/lastName combination:

import longevity.model.annotations.keyVal
import longevity.model.annotations.persistent

@keyVal[DomainModel, User]
case class Username(username: String)

@keyVal[DomainModel, User]
case class FullName(last: String, first: String)

@persistent[DomainModel]
case class User(
  username: Username,
  fullName: FullName)

object User {
  implicit val usernameKey = key(props.username)
  implicit val fullNameKey = key(props.fullName)
}

Here, no two users can have the same first and last names.

We use keys to retrieve individual persistent objects from the persistence layer, as we will see in the section on Repo.retrieve. You are most likely going to want to define at least one key per persistent type, or you will only be able to retrieve collections of persistent objects by query. It is possible that you have a persistent type - perhaps representing an entry in a log file - for which there are no natural keys. You may be satisfied to confine yourself to looking up collections of these objects via range searches.