rxkotlin groupby is not working - rx-kotlin2

Question

could you please help me to group by the following json and return a hashMap in kotlin based on date with RxKotlin? is so easy with just kotlin but really stuck for Rxkotlin. thanks
val groupedTransactions = accountTransactions.transactions
?.groupBy { it.effectiveDate }
"transactions": [{
"id": "44e5b2bc484331ea24afd85ecfb212c8",
"effectiveDate": "20/07/2017",
"description": "Kaching TFR from JOHN CITIZEN<br/>xmas donation",
"amount": 12.00
}, {
"id": "1506aeeb8c3a699b1e3c87db03156428",
"effectiveDate": "20/07/2017",
"description": "Wdl ATM CBA ATM CIRCULAR QUAY STATION NSW 221092 AUS",
"amount": -200.00,
"atmId": "129382"
}, {
"id": "9a899bfd978511e9605774e1d5222b67",
"description": "Savings",
"effectiveDate": "19/07/2017",
"amount": 10.00
}, {
"id": "1a6c48627cecaa2388b702fa33d751ff",
"description": "PTAG COCA COLA AMATI",
"effectiveDate": "12/07/2017",
"amount": -2.20
}, {
"id": "7ecc19e1a0be36ba2c6f05d06b5d3058",
"description": "Wdl ATM CBA ATM TOWN HALL SQUARE NSW 253432 AUS",
"effectiveDate": "04/07/2017",
"amount": -50.00,
"atmId": "137483"
}, {
"id": "b71bf065b640217dad602f86ac047722",
"description": "BPAY - Telstra mobile",
"effectiveDate": "04/07/2017",
"amount": -49.00
},{
"id": "ef087651eb482bae4624478696f4ad4f",
"description": "Transfer from REBECCA SHAW<br/>Lorem ipsum",
"effectiveDate": "03/07/2017",
"amount": 150.00
}, {
"id": "8cd283d8b7bacc277f2bae5e26ce6d1e",
"description": "Savings",
"effectiveDate": "01/07/2017",
"amount": 200.00
}, {
"id": "04117d2d74f5331f3ee4955da27cca7a",
"effectiveDate": "28/06/2017",
"description": "Transfer - Saturday drinks",
"amount": -100.00
}, {
"id": "821ae63dbe0c573eff8b69d451fb21bc",
"effectiveDate": "21/06/2017",
"description": "Wdl ATM CBA ATM CIRCULAR QUAY STATION NSW 221092 AUS",
"amount": -200.00,
"atmId": "129382"
}]

Answer 1

this looks lik the rxkotlin issue. I've done it through coroutine kotlin like:
doAsync { val groupedTransactions = accountTransactions.transactions ?.groupBy { it.effectiveDate } ....