Passing different object models as a parameter to a method in scala - scala

Question

I've really struggled with type relationships in scala and how to use them effectively. I am currently trying to understand how I would use them to only edit certain fields in a Mongo Collection. This means passing a particular object containing only those fields to a method which (after reading about variances) I thought that I could do like this:
abstract class DocClass
case class DocPart1(oId: Option[BSONObjectID], name: String, other: String) extends DocClass
case class DocPart2(city: String, country: String) extends DocClass
With the method that calls a more generic method as:
def updateMultipleFields(oId: Option[BSONObjectID], dataModel: DocClass): Future[Result] = serviceClientDb.updateFields[T](collectionName, dataModel, oId)
// updateFields updates the collection by passing *dataModel* into the collection, i.e. Json.obj("$set" -> dataModel)
So dataModel can be a DocPart1 or DocPart2 object. I'm eager not to use a
type parameter on updateMultipleFields (as this interesting article may suggest) as this leads me to further issues in passing these to this method in other files in the project. I'm doing this to abide with DRY and in order to maintain efficient database operations.
I've gone round in circles with this one - can anyone shed any light on this?
Edited after #SerGr's comments
So to be completely clear; I'm using Play/Scala/ReactiveMongo Play JSON (as documented here) and I have a MongoDB collection with lots of fields.
case class Doc(oId: Option[BSONObjectID], name: String, city: String, country: String, city: String, continent: String, region: String, region: String, latitude: Long, longitude: Long)
To create a new document I have auto-mapped Doc (above) to the collection structure (in Play - like this) and created a form (to insert/update the collection) - all working well!
But when editing a document; I would like to update only some fields (so that all of the fields are not updated). I have therefore created multiple case classes to divide these fields into smaller models (like the examples of DocPart1 & DocPart2) and mapped the form data to just one. This has led me to pass these as a parameter to the updateMultipleFields method as shown above. I hope that this makes more sense.

Answer 1

I'm not sure if I understand correctly what you need. Still here is some code that might be it. Assume we have our FullDoc class defined as:
case class FullDoc(_id: Option[BSONObjectID], name: String, other: String)
and we have 2 partial updates defined as:
sealed trait BaseDocPart
case class DocPart1(name: String) extends BaseDocPart
case class DocPart2(other: String) extends BaseDocPart
Also assume we have an accessor to our Mongo collection:
def docCollection: Future[JSONCollection] = ...
So if I understand your requirements, what you need is something like this:
def update[T <: BaseDocPart](oId: BSONObjectID, docPart: T)(implicit format: OFormat[T]) = {
docCollection.flatMap(_.update(BSONDocument("_id" -> oId),
JsObject(Seq("$set" -> Json.toJson(docPart)))))
}
Essentially the main trick is to use generic T <: BaseDocPart and pass implicit format: OFormat[T] so that we can convert our specific child of BaseDocPart to JSON even after type erasure.
And here is some additional test code (that I used in my console application)
implicit val fullFormat = Json.format[FullDoc]
implicit val part1Format = Json.format[DocPart1]
implicit val part2Format = Json.format[DocPart2]
def insert(id: Int) = {
val fullDoc = FullDoc(None, s"fullDoc_$id", s"other_$id")
val insF: Future[WriteResult] = docCollection.flatMap(_.insert(fullDoc))
val insRes = Await.result(insF, 2 seconds)
println(s"insRes = $insRes")
}
def loadAndPrintAll() = {
val readF = docCollection.flatMap(_.find(Json.obj()).cursor[FullDoc](ReadPreference.primaryPreferred).collect(100, Cursor.FailOnError[Vector[FullDoc]]()))
val readRes = Await.result(readF, 2 seconds)
println(s"readRes =\n${readRes.mkString("\n")}")
}
def loadRandomDocument(): FullDoc = {
val readF = docCollection.flatMap(_.find(Json.obj()).cursor[FullDoc](ReadPreference.primaryPreferred).collect(100, Cursor.FailOnError[Vector[FullDoc]]()))
val readRes = Await.result(readF, 2 seconds)
readRes(Random.nextInt(readRes.length))
}
def updateWrapper[T <: BaseDocPart](oId: BSONObjectID, docPart: T)(implicit writer: OFormat[T]) = {
val updateRes = Await.result(update(oId, docPart), 2 seconds)
println(s"updateRes = $updateRes")
}
// pre-fill with some data
insert(1)
insert(2)
insert(3)
insert(4)
val newId: Int = ((System.currentTimeMillis() - 1511464148000L) / 100).toInt
println(s"newId = $newId")
val doc21: FullDoc = loadRandomDocument()
println(s"doc21 = $doc21")
updateWrapper(doc21._id.get, DocPart1(s"p1_modified_$newId"))
val doc22: FullDoc = loadRandomDocument()
println(s"doc22 = $doc22")
updateWrapper(doc22._id.get, DocPart2(s"p2_modified_$newId"))
loadAndPrintAll()