Hi 

   I have implemented Inserting data to an SObject dynamically..... now I need to make it UPSERT functionality with given External ID field but it is throwing error as Upsert with a field specification requires a concrete SObject

Here is my Code : 

@RestResource(urlMapping='/JSONTest/*')
global with sharing class JSONtest {
@HttpPost
	global static String upsertRowDATA() {
		RestRequest req = RestContext.request;
		String datasetId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);
		String JSONString = req.requestBody.toString();
		//Schema.SObjectType custObject = Schema.getGlobalDescribe().get(datasetId+'__c');
		
		List<Object> listJsonObjects = (List<Object>)JSON.deserializeUntyped(JSONString);
		
		Schema.SObjectType soTypeObject = Schema.getGlobalDescribe().get(datasetId + '__c');
		Schema.DescribeSObjectResult objDescription = soTypeObject.getDescribe();
  		Map<String, schema.Sobjectfield> fieldsMap = objDescription.Fields.getMap();
  		system.debug(fieldsMap);
  		for(Object jsonObj : listJsonObjects) {
  			Map<String, Object> mapObj = (Map<String, Object>)jsonObj;
  			
  			system.debug(mapObj);
  			SObject sObj = Schema.getGlobalDescribe().get(datasetId + '__c').newSObject();
			
			for(String fieldName : fieldsMap.keySet()) {
			
				if(fieldName.indexOf('__c') != -1) {
					Object fieldData = mapObj.get(fieldsMap.get(fieldName).getDescribe().getName());
		   			sObj.put(fieldName, fieldData);
				}
	    	}
	    	upsert sObj ;
  		}
		 
		return JSON.serialize('{success : true}');
    }
}

 here "datasetId + '__c' " is my string type to get SObject reference.....!!!

now here in code if I write upsert sObj externalId__c;  then it gives me error that field is not a concreate SObject type....!!!

Could anyone suggest me the solution as per my requirement INSERT is working perfectly but getting problem for Updating...................... !!!

Thanks in Advance...!!

Pooja :)