Replace only repositories on a global cake pattern application

Question

I'm trying to use the cake pattern for the first time.

I kind of understand how it works, but would like to know if it is possible to mix already mixed traits or something like that.

What I would like is to build a global application with the cake pattern. And I want another version of that application which would be the same, except at the repository level.

Is it possible to do something like:

  trait application extends DefaultUserServiceComponent with MongoUserRepositoryComponent

  object realApplication extends application
  object fakeApplication extends FakeUserRepositoryComponent with application

I mean: reusing the already built application, when building a fake application using fake repositories?

Alex DiCarlo · Accepted Answer · 2013-01-10T00:52:49.980

Short answer: No. You would be inheriting conflicting members. See the following code snippet:

trait Repository {def authenticate(username: String, password: String): String}

trait UserServiceComponent {self: UserRepositoryComponent =>
  val userService: UserService = new UserService
  class UserService {
    def authenticate(username: String, password: String): String =
      repository.authenticate(username, password)
  }
}

trait UserRepositoryComponent {
  def repository: Repository
}

trait MongoUserRepositoryComponent extends UserRepositoryComponent {
  val repository: Repository =
    new Repository {def authenticate(username: String, password: String) = "MongoAuthed"}
}

trait MockUserRepositoryComponent extends UserRepositoryComponent {
  val repository: Repository =
    new Repository {def authenticate(username: String, password: String) = "MockAuthed"}
}

trait Application extends UserServiceComponent with MongoUserRepositoryComponent

object RealApplication extends Application
// The following will be an error: "object FakeApplication inherits conflicting members:"
object FakeApplication extends Application with MockUserRepositoryComponent

Instead, to have the desired behavior, define Application as such:

trait Application extends UserServiceComponent {self: UserRepositoryComponent =>}
object RealApplication extends Application with MongoUserRepositoryComponent
object FakeApplication extends Application with MockUserRepositoryComponent

To keep the hierarchy given in the OP, you would need to modify the code as such:

trait MongoUserRepositoryComponent extends UserRepositoryComponent {
  private val _repository = new Repository {def authenticate(username: String, password: String) = "MongoAuthed"}
  def repository: Repository = _repository
}

trait MockUserRepositoryComponent extends UserRepositoryComponent {
  private val _repository = new Repository {def authenticate(username: String, password: String) = "MockAuthed"}
  def repository: Repository = _repository
}

trait Application extends UserServiceComponent with MongoUserRepositoryComponent

object RealApplication extends Application
object FakeApplication extends Application with MockUserRepositoryComponent {
  override val repository: Repository = super[MockUserRepositoryComponent].repository
}

The additional private val _repository's are necessary so that we can define repository as a function, so that it can be used as an override in FakeApplication. (Using super[type] to override only works with functions).

EDIT: In the end, the purpose of the cake pattern is to develop a hierarchy where one does not need to override as in the last code snippet I provided. In reality, the trait Application should not exist at all, only RealApplication and FakeApplication. (In the 2nd code snippet I'm essentially doing nothing more than renaming UserServiceComponent to Application).

Edited to make it more "cakey" and to be more similar to http://jonasboner.com/2008/10/06/real-world-scala-dependency-injection-di/ which is a great reference for the cake pattern (and incidentally, is the example given in your question) — Alex DiCarlo, Jan 10 '13 at 00:50
Can you check this question please? :) http://stackoverflow.com/questions/14532368/cake-pattern-one-component-per-implementation-or-one-component-per-trait — Sebastien Lorber, Jan 26 '13 at 11:28

Replace only repositories on a global cake pattern application

1 Answers1