Dataset doesn't have partitioner after a repartition on one of the columns

[McBeath, Darin W (ELS-STL)]

I'm using Spark 2.0.

I've created a dataset from a parquet file and repartition on one of the 
columns (docId) and persist the repartitioned dataset.

val om = ds.repartition($"docId").persist(StorageLevel.MEMORY_AND_DISK)

When I try to confirm the partitioner, with

om.rdd.partitioner

I get

Option[org.apache.spark.Partitioner] = None

I would have thought it would be HashPartitioner.

Does anyone know why this would be None and not HashPartitioner?

Thanks.

Darin.