salesforce mobile sdk创建相关对象

Question

我有Sample_Product__c自定义对象，并希望使用关系字段的外部ID创建对象：

final Map<String, Object> prod = new HashMap<String, Object>();
prod.put("External_ID__c", obj.getString("medicineName"));

final Map < String, Object > sample = new HashMap < String, Object > ();

sample.put("Product__r", prod);
sample.put("LotNumber__c", obj.getString("medicineSerialNo"));
sample.put("GivenDate__c", format.format(givenDate));
sample.put("ExpiredDate__c", format.format(expireDate));
sample.put("GivenNumber__c", obj.getString("medicineQuantity"));
sample.put("Call__c", callid);

RestRequest.getRequestForCreate(getString(R.string.api_version), "Sample_Product__c", sample);

这会产生错误，Product__r should be SObject ...

在PHP中，我这样做是这样的：

$sfobject = new stdclass();
$sfobject->External_ID__c = "...";
$sample->Product__r = $sfobject;

但不知道如何使用salesforce mobile SDK实现这一目标...希望有人知道，谢谢！