How to implement relations using Play and Anorm

Question

I'm currently using Scala with the Play Framework and Anorm to access my database. I've been mostly a Ruby developer and still have to get used to the new concepts like dependency injection which Play uses out of the box.

I'll use a simplified example to show my problem.

Currently I have my models and persistency structured like this:

User.scala:

case class User (
  val id: Long
  val name: String
)

object User (
  val idColumn = "id"
  val nameColumn = "name"
)

UserRepository.scala:

sealed trait UserRepository {
  def findAll: List[User]
}

class DatabaseUserRepository @Inject() (db: Database) extends UserRepository {
  val parser: RowParser[User] = Macro.parser[User](
    User.idColumn,
    User.nameColumn
  )

  def findById(id: Long): List[User] = {
    db.withConnection { implicit c =>
      SQL(
        """
          SELECT * FROM users;
        """
      ).as(parser.*)
    }
  }
}

Now I have to add another model which is related to users. Let's say comments. But I struggle how to add the relation most elegantly while still using Dependency Injection.

Adding a method to the User class sounds horrible because I would always have to have an injected persistency object at the point where I call the method:

case class User (
  ...
) {
  def comments(repo: CommentRepository): List[Comment] = {
    repo.findByUserId(this.id)
  }
}

The next solution I came up with is as well far from ideal:

case class User (
  ...
) {
  var comments: List[Comment] = List()
}

class DatabaseUserRepository @Inject() (db: Database, commentRepo: CommentRepository) extends UserRepository {
  ...
  def findById(id: Long): List[User] = {
    val users = db.withConnection { implicit c =>
      SQL(
        """
          SELECT * FROM users;
        """
      ).as(parser.*)
    }

    users.map { u => u.comments = commentRepo.findByUserId(u.id) }
  }
}

This way I would always fetch the comments and not when I need them AND generally the idea of using an empty var comments list and later filling it feels wrong.

How would it look like to structure relations between models while preserving immutable state and dependency injection? Are there some examples anywhere?

Thanks!

Why would such "repository" dependencies be needed? Common functions and rows parser could be shared. — cchantep
@cchantep I am open for new suggestions. Do you have an example of good structured models and persistency with relations? — Chris
@cchantep I am sorry but I am not sure what you mean. I somehow need a persistency class in which I inject a Database instance given by the Play framework no? In this case I called these objects repositories — Chris
@cchantep It would be wonderful if you could post an answer showing an example with what you have in mind. Thanks ! — Chris

airudah airudah · Accepted Answer · 2018-02-08T17:21:52

You can create an option of comments in the user object:

case class User(id: Long, name: String, comments: Option[Seq[Comments]])

Then build the object from your query:

def findById(id: Long): List[User] = {
    val users = db.withConnection { implicit c =>
      SQL("SELECT u.id, u.name, c.comment FROM users as u 
           join Comments as c on u.id = c.id 
           where u.id = {uid}")
          .on('uid -> id)
          .as(parser.*)
    }

You can then work out how to get a RowParser to create a None where no comments exist.

Have a look at Anorm Row Parser API for some inspiration

How to implement relations using Play and Anorm

1 Answers