for (let i=0; i<30000; ++i) {
    let doc = {}
    if (i % 2 === 0) doc._id = 'id_' + i.toString()
    doc.index = i
    db.foo.insert(doc)
}

import com.mongodb.spark.MongoSpark
import com.mongodb.spark.config.ReadConfig
import org.apache.spark.sql.SparkSession

case class MyRecord(
  index: Double
)

object Main {
  def main(args: Array[String]) = {
    val spark = SparkSession.builder()
      .appName("sample")
      .master("local[2]")
      .getOrCreate()

    import spark.implicits._

    val ds = MongoSpark.load[MyRecord](spark, ReadConfig(
      Map(
        "collection" -> "foo",
        "uri" -> "mongodb://localhost/mydb",
        "partitioner" -> "MongoSamplePartitioner",
        "partitionerOptions.partitionSizeMB" -> "1")
      )
    ).as[MyRecord]

    println("total: " + ds.count())
    println("evens (typeof _id = string): " + ds.filter(_.index % 2 == 0).count())
    println("odds (typeof _id = ObjectId): " + ds.filter(_.index % 2 == 1).count())
  }
}