Groovy collect from map and submap - json

Question

I had a requirement to convert a JSON response into a csv file. I was able to successfully use Tim Yates' excellent code from here: Groovy code to convert json to CSV file
I now need to include the JSON's nested submap in the csv as well. The relationship between the map and submap is 1:1.
I've been unable to get the correct syntax for a collect statement that will retrieve both the parsed map and submap key/values.
Sample JSON
{items=
[
{
created_at=2019-03-27
, entity_id=1
, extension_attributes=[]
},
{
created_at=2019-03-27
, entity_id=2
, extension_attributes= { employee_id=Emp1, employee_type=CSR}//nested submap
}
]}
Groovy
import groovy.json.*
def data = new JsonSlurper().parseText( json ); //"json" is from GET request
def columns = ["created_at","entity_id","employee_id","employee_type"]
def encode = { e -> e ? /"$e"/ : "$e"}
requestFile.append( columns.collect { c -> encode( c ) }.join( ',' ) + '\n');
requestFile.append( data.items.collect { row ->columns.collect { colName -> encode( row[ colName ] ).replaceAll("null","") }.join( ',' )}.join( '\n' ) );//unsure how to use data.items.collect to fetch submap
I would like to either
1) Convert the JSON as follows to collect each key/value easily:
...
{
created_at=2019-03-27
, entity_id=2
, employee_id=Emp1
, employee_type=CSR
}
...
or 2) Find out if there's a way to use Groovy's collect method to retrieve the map/submap as a flat map.
I am unfortunately not a programmer by trade, and any help would be appreciated!

Answer 1

Here is the flatten closure which flattens an item recursively:
def flatten
flatten = { row ->
def flattened = [:]
row.each { k, v ->
if (v instanceof Map) {
flattened << flatten(v)
} else {
flattened[k] = v
}
}
flattened
}
You should just replace row with flatten(row) at your last line so it looks like this:
requestFile.append(data.items.collect { row ->
columns.collect {
colName -> encode(flatten(row)[colName]).replaceAll("null", "")
}.join(',')
}.join('\n'))
The result will be as follows:
"created_at","entity_id","employee_id","employee_type"
"2019-03-27","1",,
"2019-03-27","2","Emp1","CSR"

Answer 2

Also found that following allows for collect method to fetch nested elements:
def m = data.items.collect{[/"${it?.created_at?:''}"/,/"${it?.extension_attributes?.entity_id?:''}"/,/"${it?.extension_attributes?.employee_id?:''}"/,/"${it?.extension_attributes?.employee_type?:''}"/]}
m.each{requestFile.append(it.join(',')+'\n')}