Structs as values in maps not being properly read from Parquet files

Given the following example structure

val childStructType = StructType(
Field("childFoo", StringType),
Field("childBar", ArrayType(StringType), nullable = true)
)
val parentStructType = StructType(
Field("foo", StringType),
Field("children", MapType(StringType, childStructType))
)

persisting a row to a Parquet file works fine but reading the content returns null values in the child structure fields. The following is a full class to reproduce the issue:

import io.eels.component.parquet.{ParquetSink, ParquetSource}
import io.eels.datastream.DataStream
import io.eels.schema._
import org.apache.hadoop.conf.Configuration
import org.apache.hadoop.fs.{FileSystem, Path}

object EelBugReproductionTest extends App {

private implicit val conf = new Configuration()
private implicit val fs = FileSystem.get(new Configuration())

case class Child(foo: String, bar: Array[String]) {
override def toString: String = {
s"$foo :: ${bar.mkString(",")}"
}
}
case class Parent(foo: String, children: Map[String, Child]) {
override def toString: String = {
s"$foo :: ${children.mkString(" >> ")}"
}
}

val childStructType = StructType(
Field("foo", StringType),
Field("bar", ArrayType(StringType))
)
val parentStructType = StructType(
Field("foo", StringType),
Field("children", MapType(StringType, childStructType))
)

def toSeq(c: Child): Seq[Any] = Seq(c.foo, c.bar)

def toDataStream(p: Parent) = {
val values = Seq(p.foo, p.children.mapValues(toSeq))
DataStream.fromValues(parentStructType, Seq(values))
}

def write(p: Parent, path: Path): Unit = {
if (fs.exists(path)) fs.delete(path, false)
toDataStream(p).to(ParquetSink(path))
}

def toChild(values: Seq[Any]): Child = {
val foo = values(0).asInstanceOf[String]
val bar = values(1).asInstanceOf[Vector[String]].toArray
Child(foo, bar)
}

def readParent(path: Path): Parent = {
val ps = ParquetSource(path)
val row = ps.toDataStream.head
val foo = row.values(0).asInstanceOf[String]
val children = row.values(1).asInstanceOf[Map[String, Seq[Any]]].mapValues(toChild)
Parent(foo, children)
}

val path = new Path("test.pq")
if (fs.exists(path)) fs.delete(path, false)

val child1 = Child("foo1", Array("bar11", "bar12"))
val child2 = Child("foo2", Array("bar21", "bar22"))
val parent = Parent("foo1", Map("child1" -> child1, "child2" -> child2))

write(parent, path)
println(readParent(path))

}

I have managed to fix the issue locally by amending RowReadSupport.scala as per the following diff:

diff --git a/eel-core/src/main/scala/io/eels/component/parquet/RowReadSupport.scala b/eel-core/src/main/scala/io/eels/component/parquet/RowReadSupport.scala
index 7ec2501e..ebdb32a3 100644
--- a/eel-core/src/main/scala/io/eels/component/parquet/RowReadSupport.scala
+++ b/eel-core/src/main/scala/io/eels/component/parquet/RowReadSupport.scala
@@ -132,11 +132,13 @@ class MapConverter(index: Int,

private val keys = new VectorBuilder()
private val values = new VectorBuilder()
+ private val keysConverter = Converter(mapType.keyType, false, -1, keys)
+ private val valuesConverter = Converter(mapType.valueType, false, -1, values)

override def getConverter(fieldIndex: Int): Converter = new GroupConverter {
override def getConverter(fieldIndex: Int): Converter = fieldIndex match {
- case 0 => Converter(mapType.keyType, false, -1, keys)
- case 1 => Converter(mapType.valueType, false, -1, values)
+ case 0 => keysConverter
+ case 1 => valuesConverter
}
override def start(): Unit = ()
override def end(): Unit = () // a no-op as each nested group only contains a single element and we want to handle the finished list
~

I'd appreciate if you could take a look both at the issue and my amendment and merge it if it makes sense to you. Thanks

Regards,
Iñaki

The issue is in RowReadSupport.scala, specifically the MapConverter class. The fix involves reusing converters for map keys and values. Start by examining the eel-core/src/main/scala/io/eels/component/parquet/RowReadSupport.scala file around line 132. Understand how MapConverter works and why the current implementation fails for nested structs in maps. Run the provided reproduction test to see the null values, then apply the diff and verify the fix works.

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