[SPARK-14159][ML] Fixed bug in StringIndexer + related issue in RFormula

## What changes were proposed in this pull request?

StringIndexerModel.transform sets the output column metadata to use name inputCol.  It should not.  Fixing this causes a problem with the metadata produced by RFormula.

Fix in RFormula: I added the StringIndexer columns to prefixesToRewrite, and I modified VectorAttributeRewriter to find and replace all "prefixes" since attributes collect multiple prefixes from StringIndexer + Interaction.

Note that "prefixes" is no longer accurate since internal strings may be replaced.

## How was this patch tested?

Unit test which failed before this fix.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #11965 from jkbradley/StringIndexer-fix.

1 files changed, 13 insertions, 0 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
index d40e69dced..2c3255ef33 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
@@ -210,4 +210,17 @@ class StringIndexerSuite
       .setLabels(Array("a", "b", "c"))
     testDefaultReadWrite(t)
   }
+
+  test("StringIndexer metadata") {
+    val data = sc.parallelize(Seq((0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c")), 2)
+    val df = sqlContext.createDataFrame(data).toDF("id", "label")
+    val indexer = new StringIndexer()
+      .setInputCol("label")
+      .setOutputCol("labelIndex")
+      .fit(df)
+    val transformed = indexer.transform(df)
+    val attrs =
+      NominalAttribute.decodeStructField(transformed.schema("labelIndex"), preserveName = true)
+    assert(attrs.name.nonEmpty && attrs.name.get === "labelIndex")
+  }
 }