How to unwind array in DataFrame (from JSON)? - scala

Question

Each record in an RDD contains a json. I'm using SQLContext to create a DataFrame from the Json like this:
val signalsJsonRdd = sqlContext.jsonRDD(signalsJson)
Below is the schema. datapayload is an array of items. I want to explode the array of items to get a dataframe where each row is an item from datapayload. I tried to do something based on this answer, but it seems that I would need to model the entire structure of the item in the case Row(arr: Array[...]) statement. I'm probably missing something.
val payloadDfs = signalsJsonRdd.explode($"data.datapayload"){
case org.apache.spark.sql.Row(arr: Array[String]) => arr.map(Tuple1(_))
}
The above code throws a scala.MatchError, because the type of the actual Row is very different from Row(arr: Array[String]). There is probably a simple way to do what I want, but I can't find it. Please help.
Schema give below
signalsJsonRdd.printSchema()
root
|-- _corrupt_record: string (nullable = true)
|-- data: struct (nullable = true)
| |-- dataid: string (nullable = true)
| |-- datapayload: array (nullable = true)
| | |-- element: struct (containsNull = true)
| | | |-- Reading: struct (nullable = true)
| | | | |-- A2DPActive: boolean (nullable = true)
| | | | |-- Accuracy: double (nullable = true)
| | | | |-- Active: boolean (nullable = true)
| | | | |-- Address: string (nullable = true)
| | | | |-- Charging: boolean (nullable = true)
| | | | |-- Connected: boolean (nullable = true)
| | | | |-- DeviceName: string (nullable = true)
| | | | |-- Guid: string (nullable = true)
| | | | |-- HandsFree: boolean (nullable = true)
| | | | |-- Header: double (nullable = true)
| | | | |-- Heading: double (nullable = true)
| | | | |-- Latitude: double (nullable = true)
| | | | |-- Longitude: double (nullable = true)
| | | | |-- PositionSource: long (nullable = true)
| | | | |-- Present: boolean (nullable = true)
| | | | |-- Radius: double (nullable = true)
| | | | |-- SSID: string (nullable = true)
| | | | |-- SSIDLength: long (nullable = true)
| | | | |-- SpeedInKmh: double (nullable = true)
| | | | |-- State: string (nullable = true)
| | | | |-- Time: string (nullable = true)
| | | | |-- Type: string (nullable = true)
| | | |-- Time: string (nullable = true)
| | | |-- Type: string (nullable = true)

Answer 1

tl;dr explode function is your friend (or my favorite flatMap).
explode function creates a new row for each element in the given array or map column.
Something like the following should work:
signalsJsonRdd.withColumn("element", explode($"data.datapayload"))
See functions object.