Three level nested structure flattening fails

[gorros]

When I add one more level of nested structure it fails to flatten.

"uses the StackOverflow answer format" - {

        val data = Seq(
          Row(
            Row(
              "this",
              "is"
            ),
            "something",
            "cool",
            ";)"
          )
        )

        val schema = StructType(
          Seq(
            StructField(
              "foo",
              StructType(
                Seq(
                  StructField(
                    "bar",
                    StructType(
                      Seq(
                        StructField(
                          "zoo",
                          StringType,
                          true
                        )
                      )
                    )
                  ),
                  StructField(
                    "baz",
                    StringType,
                    true
                  )
                )
              ),
              true
            ),
            StructField(
              "x",
              StringType,
              true
            ),
            StructField(
              "y",
              StringType,
              true
            ),
            StructField(
              "z",
              StringType,
              true
            )
          )
        )

        val df = spark
          .createDataFrame(
            spark.sparkContext.parallelize(data),
            StructType(schema)
          )
          .flattenSchema("_")

        val expectedDF = spark.createDF(
          List(("this", "is", "something", "cool", ";)")),
          List(
            ("foo_bar_zoo", StringType, true),
            ("foo_baz", StringType, true),
            ("x", StringType, true),
            ("y", StringType, true),
            ("z", StringType, true)
          )
        )

        assertSmallDataFrameEquality(
          df,
          expectedDF
        )

      }