declaring a domain model

The first step in building out our domain model is to define a trait that describes our model. This trait never needs to be instantiated. It is used as a type parameter to identify the elements of our model, and the tools that longevity provides for you, as being specific to this model. This is done to provide a strong degree of type safety.

We annotate this domain model trait with the longevity.model.annotations.domainModel annotation:

package myPackage

import longevity.model.annotations.domainModel

@domainModel trait MyDomainModel

The details of what the @domainModel annotation should not be important to the casual user. We present them here for the sake of completeness, for interested readers, and in case you end up needing to troubleshoot a problem. If you do find yourself troubleshooting, be sure to give us a shout on Gitter or the user group. We are eager to help!

Like all the other model annotations in longevity, it is possible to write out the equivalent code yourself. We don’t recommend that you do this. It is a lot of boilerplate.

The @domainModel annotation creates or extends the companion object of your trait, adding in a couple of implicit type classes, like so:

package myPackage

import longevity.model.CType
import longevity.model.KVType
import longevity.model.ModelEv
import longevity.model.ModelType
import longevity.model.PType
import longevity.model.annotations.packscanToList

trait MyDomainModel

object MyDomainModel {

  private[myPackage] implicit object modelEv extends ModelEv[MyDomainModel]

  implicit object modelType extends ModelType[MyDomainModel](
    packscanToList[PType[MyDomainModel, _]],
    packscanToList[CType[MyDomainModel, _]],
    packscanToList[KVType[MyDomainModel, _, _]])
}

The ModelEv[MyDomainModel] is evidence of the model. The compiler needs to be able to locate this evidence when constructing a persistent, component, or a key value. All your persistent classes need to be found in the same package as, or a subpackage of, the package that you define your domain model. Scoping the implicit model evidence this way makes it findable by implicit resolution in exactly the right places.

The ModelType[MyDomainModel] collects information about all of your persistents, components, and key values. This ModelType is used by the longevity context to provide you with tools specific to your model. It will be found automatically by implicit resolution when constructing your context like so: longevity.context.LongevityContext[MyDomainModel].

packscanToList is a def macro that scans the current package, and all sub-packages, for objects that match the provided types. For instance, if you had declared persistent classes User, Blog, and BlogPost, a component UserProfile, and key values Username, BlogUri, and BlogPostUri, then the above code would expand as follows:

package myPackage

import longevity.model.ModelEv
import longevity.model.ModelType

trait MyDomainModel

object MyDomainModel {

  private[myPackage] implicit object modelEv extends ModelEv[MyDomainModel]

  implicit object modelType extends ModelType[MyDomainModel](
    List(User, Blog, BlogPost),
    List(UserProfile),
    List(Username, BlogUri, BlogPostUri))
}