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Looking for suggestions on how to unit test a Spark transformation with ScalaTest. The test class generates a DataFrame from static data and passes it to a transformation, then makes assertion on the passing static data generated in the test class. (The transform creates a second column b defined as col("a").plus(5).)

I got this to work but wonder if there's a better way to create the DataFrame? asJava feels awkward, as does defining the row data and schema separately.

class TransformTest extends FlatSpec with Matchers with SharedSparkContext {
  "Transformer" should "add column to dataframe" in {
    val sqlContext = new SQLContext(sc)
    val rows = Seq[Row](
      Row(1),
      Row(2),
      Row(3)
     ).asJava
   val schema = StructType(Seq(StructField("a", IntegerType)))

   val df = sqlContext.createDataFrame(rows, schema)
   val df2 = new Transform().addCol(df)
   assert(df2.count() > 0)
   assert(df2.agg(sum("a")).first.getLong(0) == 6)
   assert(df2.agg(sum("b")).first.getLong(0) == 21)
  }
}
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Indeed, asJava looks awkward and completely unnecessary here. You can approach this problem in a few different ways.

  • pass RDD[Row] instead of Seq[Row] to avoid asJava:

    sqlContext.createDataFrame(sc.parallelize(Seq(Row(1), Row(2), Row(3))), schema
    
  • use product types instead of Row to avoid both asJava and passing schema:

    sqlContext.createDataFrame(Seq(1, 2,  3).map(Tuple1(_))).toDF("a")
    

    or

    case class Record(x: Int)
    
    sqlContext.createDataFrame(Seq(1, 2,  3).map(Record(_)))
    
  • use implicit conversions to avoid asJava and passing schema and get concise syntax (supported for Seq[Int] only in Spark 2.0+):

    import spark.implicits._
    
    Seq(1, 2, 3).toDF("a")
    

In general you should prefer product types (tuples, case classes) over Row objects. You get schema for free as well as some static typing.

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  • \$\begingroup\$ Sorry, what do you mean by "product type"? \$\endgroup\$ – wrschneider Aug 18 '16 at 0:56
  • \$\begingroup\$ From theoretical perspective see Wikipedia, from practical subclasses of scala.Product. \$\endgroup\$ – zero323 Aug 18 '16 at 15:50

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