ReactiveMongo: How to deserialize a list into an O

问题:

Here below is a case class that represents an user, and its companion object provides a BSONDocumentWriter and BSONDocumentReader to serialize/deserialize to/from BSON:

case class User(
  id: Option[BSONObjectID],
  name: String,
  addresses: Option[List[BSONObjectID]]
)

object User {
  implicit object UserWriter extends BSONDocumentWriter[User] {
    def write(user: User) = BSONDocument(
      "_id" -> user.id.getOrElse(BSONObjectID.generate),
      "name" -> user.name,
      "addresses" -> user.addresses
    ) 
  }

  implicit object UserReader extends BSONDocumentReader[User] {
    def read(doc: BSONDocument) = User(
      doc.getAs[BSONObjectID]("_id"),
      doc.getAs[String]("name").get,
      doc.getAs[List[BSONObjectID]]("addresses").toList.flatten,
    )
  }
}

The code above does not compile because User.addresses is an Option and I always get the following error:

/home/j3d/Projects/test/app/models/User.scala:82: polymorphic expression cannot be instantiated to expected type;
[error]  found   : [B]List[B]
[error]  required: Option[List[reactivemongo.bson.BSONObjectID]]
[error]       doc.getAs[List[BSONObjectID]]("addresses").toList.flatten,
                                                                ^

If the input BSON contains addresses, how do I deserialize it into an Option[List[BSONObjectID]]?

回答1:

Why are you calling toList and then flatten? If I'm reading the docs correctly for BSONDocument, the getAs method will return an Option for type T where in your case, you are specifying T as List[BSONObjectID]. When you call toList, you are basically unpacking the Option to either an empty List (in the case of a None) or into the actual List in the case of a Some. And then the flatten here doesn't seem to be doing anything relevant. Can't you just do it as:

doc.getAs[List[BSONObjectID]]("addresses")