In Lift with MongoDB, storing lists of heterogeneous data

Question

I need to make a web service that can will contain lists of objects. One list can contain objects of many types. Here, for example, I have a library of media items. Each item can be either a link or a video, each with their own metadata.

I want to do this with the Lift web framework, because I need something that compiles to WAR and I've used Lift before.

I thought that using MongoDB for this sort of storage would work as, by definition, it's supposed to be able to handle collections of heterogeneous items.

I can define types of BSON objects to be stored in Lift records, but I can't seem to stick away from creating only one type of object in one record/collection. Ideally, I'd like each "thing" (for lack of a better word) in my Library to be either a video or a link. For example:

[
  {
    "type"       : "Video",
    "title"      : "Story",
    "videoID"    : "123ab4",
    "description": "Feature-length epic",
    "time"       : 12.6
  },
  {
    "type" : "link",
    "url"  : "http://www.google.com",
    "title": "Search for stuff"
  }
]

I should be able to do it with the right type of inheritance, but the way all of the record objects's parents inherit from the object throws me off. Can I get this to work? Having a collection of different things that Lift can use as such?

Here's what I have so far. I haven't tested it, but even if it works what it does is NOT what I want.

import net.liftweb.record._
import net.liftweb.record.field._

import net.liftweb.mongodb._
import net.liftweb.mongodb.record._
import net.liftweb.mongodb.record.field._


class VideoShelf private () extends BsonRecord[VideoShelf] {
  def meta = VideoShelf

  object title       extends StringField (this, 256)
  object videoID     extends StringField (this, 32 )
  object description extends StringField (this, 256)
  object time        extends DecimalField(this, 0  )
}

object VideoShelf extends VideoShelf with BsonMetaRecord[VideoShelf]


class LinkShelf private () extends BsonRecord[LinkShelf] {
  def meta = LinkShelf

  object url   extends StringField(this, 128)
  object title extends StringField(this, 256)
}

object LinkShelf extends LinkShelf with BsonMetaRecord[LinkShelf]


class MediaLibrary private () extends MongoRecord[MediaLibrary] with ObjectIdPk[MediaLibrary] {
  def meta = MediaLibrary

  ///////////////////////////////////////
  ///////////////////////////////////////
  // What I want is this record type to
  // contain either of these:
  ///////////////////////////////////////      
  object videoshelf extends BsonRecordField(this, VideoShelf)
  object linkshelf  extends BsonRecordField(this, LinkShelf )
}

object MediaLibrary extends MediaLibrary with MongoMetaRecord[MediaLibrary]

How can I get this?

Answer 1

After seeking more, I found this post: https://groups.google.com/forum/#!topic/liftweb/LmkhvDgrgrI

That led me to this conclusion, which I think is correct, though it has not been tested yet. I may be missing some pieces for it to fully work.

import net.liftweb.record._
import net.liftweb.record.field._

import net.liftweb.mongodb._
import net.liftweb.mongodb.record._
import net.liftweb.mongodb.record.field._


/**
 * The base record type for library objects.
 */
trait MediaLibrary[T <: MediaLibrary[T]] extends MongoRecord[T] with ObjectIdPk[T] {
  self: T =>
}

/**
 * Items in the library that are videos.
 */
class VideoItem extends MediaLibrary[VideoItem] {

  def meta = VideoItem

  object title       extends StringField (this, 256)
  object videoID     extends StringField (this, 32 )
  object description extends StringField (this, 256)
  object time        extends DecimalField(this, 0  )
}

object VideoItem extends VideoItem with MongoMetaRecord[VideoItem]

/**
 * Items in the library that are links.
 */
class LinkItem extends MediaLibrary[LinkItem] {

  def meta = LinkItem

  object url         extends StringField (this, 256)
  object title       extends StringField (this, 256)    
}

object LinkItem extends LinkItem with MongoMetaRecord[LinkItem]

UPDATE

I've also just found out that there is a MongoDB-specific record that's a list of case classes. That seems to be exactly what I need! It's the power of Scala and Mongo being used hand in hand! That's what I wanted from the start. I'll have to try that tomorrow.