[SPARK-9463] [ML] Expose model coefficients with names in SparkR RFormula

Preview:

```
> summary(m)
            features coefficients
1        (Intercept)    1.6765001
2       Sepal_Length    0.3498801
3 Species.versicolor   -0.9833885
4  Species.virginica   -1.0075104

```

Design doc from umbrella task: https://docs.google.com/document/d/10NZNSEurN2EdWM31uFYsgayIPfCFHiuIu3pCWrUmP_c/edit

cc mengxr

Author: Eric Liang <ekl@databricks.com>

Closes #7771 from ericl/summary and squashes the following commits:

ccd54c3 [Eric Liang] second pass
a5ca93b [Eric Liang] comments
2772111 [Eric Liang] clean up
70483ef [Eric Liang] fix test
7c247d4 [Eric Liang] Merge branch 'master' into summary
3c55024 [Eric Liang] working
8c539aa [Eric Liang] first pass

2 files changed, 22 insertions, 4 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/OneHotEncoderSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/OneHotEncoderSuite.scala
index 65846a846b..321eeb8439 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/OneHotEncoderSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/OneHotEncoderSuite.scala
@@ -86,8 +86,8 @@ class OneHotEncoderSuite extends SparkFunSuite with MLlibTestSparkContext {
     val output = encoder.transform(df)
     val group = AttributeGroup.fromStructField(output.schema("encoded"))
     assert(group.size === 2)
-    assert(group.getAttr(0) === BinaryAttribute.defaultAttr.withName("size_is_small").withIndex(0))
-    assert(group.getAttr(1) === BinaryAttribute.defaultAttr.withName("size_is_medium").withIndex(1))
+    assert(group.getAttr(0) === BinaryAttribute.defaultAttr.withName("small").withIndex(0))
+    assert(group.getAttr(1) === BinaryAttribute.defaultAttr.withName("medium").withIndex(1))
   }
 
   test("input column without ML attribute") {
@@ -98,7 +98,7 @@ class OneHotEncoderSuite extends SparkFunSuite with MLlibTestSparkContext {
     val output = encoder.transform(df)
     val group = AttributeGroup.fromStructField(output.schema("encoded"))
     assert(group.size === 2)
-    assert(group.getAttr(0) === BinaryAttribute.defaultAttr.withName("index_is_0").withIndex(0))
-    assert(group.getAttr(1) === BinaryAttribute.defaultAttr.withName("index_is_1").withIndex(1))
+    assert(group.getAttr(0) === BinaryAttribute.defaultAttr.withName("0").withIndex(0))
+    assert(group.getAttr(1) === BinaryAttribute.defaultAttr.withName("1").withIndex(1))
   }
 }
diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/RFormulaSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/RFormulaSuite.scala
index 8148c553e9..6aed3243af 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/RFormulaSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/RFormulaSuite.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.ml.feature
 
 import org.apache.spark.SparkFunSuite
+import org.apache.spark.ml.attribute._
 import org.apache.spark.ml.param.ParamsSuite
 import org.apache.spark.mllib.linalg.Vectors
 import org.apache.spark.mllib.util.MLlibTestSparkContext
@@ -105,4 +106,21 @@ class RFormulaSuite extends SparkFunSuite with MLlibTestSparkContext {
     assert(result.schema.toString == resultSchema.toString)
     assert(result.collect() === expected.collect())
   }
+
+  test("attribute generation") {
+    val formula = new RFormula().setFormula("id ~ a + b")
+    val original = sqlContext.createDataFrame(
+      Seq((1, "foo", 4), (2, "bar", 4), (3, "bar", 5), (4, "baz", 5))
+    ).toDF("id", "a", "b")
+    val model = formula.fit(original)
+    val result = model.transform(original)
+    val attrs = AttributeGroup.fromStructField(result.schema("features"))
+    val expectedAttrs = new AttributeGroup(
+      "features",
+      Array(
+        new BinaryAttribute(Some("a__bar"), Some(1)),
+        new BinaryAttribute(Some("a__foo"), Some(2)),
+        new NumericAttribute(Some("b"), Some(3))))
+    assert(attrs === expectedAttrs)
+  }
 }