Alpakka MongoDB - specify type in MongoSource

Question

I'm currently playing around with Akka Streams and the Alpakka MongoDB connector.

Is it possible to specify the type for MongoSource?

val codecRegistry = fromRegistries(fromProviders(classOf[TodoMongo]), DEFAULT_CODEC_REGISTRY)
  private val todoCollection: MongoCollection[TodoMongo] = mongoDb
    .withCodecRegistry(codecRegistry)
    .getCollection("todo")

I would like to do something like this:

val t: FindObservable[Seq[TodoMongo]] = todoCollection.find()
MongoSource(t) // Stuck here

But I get the following error:

Expected Observable[scala.Document], Actual FindObservable[Seq[TodoMongo]].

I can't find the correct documentation about this part.

`MongoSource(todoCollection.find())` is straight forward. what do you want to do with the intermediate `FindObservable `? — Knows Not Much, Mar 10 '18 at 14:32
Or you can have a look at http://reactivemongo.org/releases/0.1x/documentation/tutorial/streaming.html — cchantep, Mar 10 '18 at 14:43

Jeffrey Chung · Accepted Answer · 2018-03-12T15:47:13.697

This is not published yet, but in Alpakka's master branch, MongoSource.apply takes a type parameter:

object MongoSource {
  def apply[T](query: Observable[T]): Source[T, NotUsed] =
    Source.fromPublisher(ObservableToPublisher(query))
}

Therefore, with the upcoming 0.18 release of Alpakka, you'll be able to do the following:

val source: Source[TodoMongo, NotUsed] = MongoSource[TodoMongo](todoCollection.find())

Note that source here assumes that todoCollection.find() returns an Observable[TodoMongo]; adjust the types as needed.

In the meantime, you could simply add the above code manually. For example:

package akka.stream.alpakka.mongodb.scaladsl

import akka.NotUsed
import akka.stream.alpakka.mongodb.ObservableToPublisher
import akka.stream.scaladsl.Source
import org.mongodb.scala.Observable

object MyMongoSource {
  def apply[T](query: Observable[T]): Source[T, NotUsed] =
    Source.fromPublisher(ObservableToPublisher(query))
}

Note that MyMongoSource is defined to reside in the akka.stream.alpakka.mongodb.scaladsl package (like MongoSource), because ObservableToPublisher is a package-private class. You would use MyMongoSource in the same way that you would use MongoSource:

val source: Source[TodoMongo, NotUsed] = MyMongoSource[TodoMongo](todoCollection.find())

Great answer. Was looking for something like this. But how to add the type manually with the following apply method of MongoSource? def apply(query: Observable[Document]): Source[Document, NotUsed] — , Mar 10 '18 at 15:38

Alpakka MongoDB - specify type in MongoSource

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