Allow vectortile.Layer to pass a valid 'id' to internal.vector_tile.Tile.Layer

[kevinmhinson]

Hi! I'm using GeoTrellis and VectorPipe to create MapBox vector tiles. The problem I have is... I need to be able to write a top level 'id' in each Feature (not as part of 'properties')... and in vectortile.Layer.unfeature(), the arg id: Option[Long] passed to vectortile.internal.vector_tile.Tile.Feature is hard-coded in .unfeature() as None.

To solve this for my purposes, I forked and compiled locally with a Map[String,Value] => Option[Long] function hard coded in Layer.unfeature() to create an 'id' from 'data' to pass to vt.Tile.Feature. This worked, and the output .PBFs had the expected 'id' in them.

I'm trying to think through how else users might want to create an 'id'... maybe a random Long would be sufficient for some, but that could still be coded into a 'data' => 'id' function.

I believe this would be a common pattern, to want an 'id' and to be able to create from the metadata. It looks like the VectorPipe API has completely changed since two days ago, but it used to be in Collate.generically() where a vectortile.StrictLayer is constructed. If there was a way to pass in a Map[String,Value] => Option[Long] function to Layer/StrictLayer/LazyLayer for Layer to use in .unfeature() to convert 'data' to valid 'id' instead of hard-coding as None... I think that would be a common use case.

[jpolchlo]

Looks like you're absolutely right. The GeoJSON spec indeed does include an optional id property. For reference, https://tools.ietf.org/html/rfc7946#page-11 shows that this identifier should be placed into a separate top-level field of the feature. Given that, it's not clear that the right solution is simply to allow properties to be mined for this data, since that may imply that the id field will be redundantly serialized to both the properties and id field. We may want to include a proper id field in Feature[G, D] itself and let that change propagate into vectortile naturally.

Any thoughts, @echeipesh, @pomadchin ?

[echeipesh]

A breadcrumb for where VectorTile feature are being created:

geotrellis/vectortile/src/main/scala/geotrellis/vectortile/Layer.scala

Lines 127 to 140 in f8af6e6

	private def unfeature(
	keys: Map[String, Int],
	values: Map[Value, Int],
	geomType: vt.Tile.GeomType,
	cmds: Seq[Command],
	data: Map[String, Value]
	): vt.Tile.Feature = {
	val tags = data.toSeq.foldRight(List.empty[Int]) { case (pair, acc) =>
	/* These `Option.get` _should_ never fail */
	keys.get(pair._1).get :: values.get(pair._2).get :: acc
	}
	vt.Tile.Feature(None, tags, Some(geomType), Command.uncommands(cmds))
	}