problemi elementari - connessioni ai db nei funzionali

[ildella]

Ciao a tutti.

Il problema e' banale ma evidentemente mi manca un sacco di contesto lavorando con i funzionali.

Seguendo la doc di ReactiveMongo, ma credo il problema sia simile per qualsiasi db:
https://github.com/ReactiveMongo/ReactiveMongo
loro fanno un bravo metodo connect() che alla fine mi porta ad avere la collection.
poi la collection la uso in un altro metodo, per leggere o scrivere.

ora in questo secondo metodo, la collection dovrebbe essere una, orrore, variabile di classe? In Java o simili sarebbe stato cosi'.
Qui invece come dovrei fare?

[Shirish Padalkar]

Hi,

I have used Google Translator for translation, so please pardon me if I misunderstood your question.

If I understood you correctly, you are asking if collections should be class variables.

Play-reactivemongo actually recommends it to be a def instead of val. You can refer the documentation here - https://github.com/ReactiveMongo/Play-ReactiveMongo

Otherwise, if you are not using it inside Play application, I don't see any problems having it as a private val inside a class.

​

With regards,
Shirish (http://twitter.com/_Garbage_)

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[ildella]

Hi,

that was my point, I do not want to make it a class variable but I do not know how to write it properly using a val.
Now if I can read properly, the line:

  def collection: JSONCollection = db.collection[JSONCollection]("persons")

db is injected somehow from the application.conf
declaring "def collection" is what makes the trick to have the "collection" variable always available in the code. 

my problem was very level, like how can I access a val in different method without having to call all the connection.db and db.collection all the time... 
Probably the driver is smart enough... Anyway I'll follow that  thanks. 

[Marco F.]

What I'd do is have connection/collection being handled by an Object.

Something along these lines:

object MongoHelper {
  import reactivemongo.api._
  import scala.concurrent.ExecutionContext.Implicits.global

  val driver = new MongoDriver
  val connection = driver.connection(List("localhost"))

  val db = connection("my-db") 
  val collectionA = db("acoll")
}

So then I can just import the Object from somewhere else and access collectionA from it.

How does that look to you guys?

-marco

[Shirish Padalkar]

Well, that sounds okay.

My worry is, it unnecessarily presents db, driver and connection to the client. While I understand there might be ways to get to the driver from collectionA, I think these are more details than required for the client which just want to have access to collection. Of course, it can be solved by making them private. :)

When I am using dependencies injection, my approach is almost same. I declare it a singleton class with all internal variables private and only expose defs for collections. This object can then be injected to any controller / service easily.

Another approach is using cake-pattern, but I think it's too much hassle. ;)

With regards,
Shirish (http://twitter.com/_Garbage_)

[Diego]

using cake pattern something like this I think:

trait ConnectionComponent {

   def connection: Connection

}

trait ConnectionComponentImpl extends ConnectionComponent {

   lazy val connection: Connection = { ..IMPL } //you can override def with val or keep def

}

trait UserServiceImpl extends UserService { this : ConnectionComponent =>

    def findUser(id:Long) = //IMPL and access to connection

}

object Application {

     val app = new UserServiceImpl with ConnectionComponentImpl

     

     app.findUser(1l) //OK