My Webservice GET's a list of records in JSON format from another system, then updates to those specific records in Salesforce.
My data model: I have a master detail relationship between Account & Advance_Summary__c custom object. RC_Account_No__c is an external ID field that's unique on Account and Balance__c  is a custom currency field on Advance_Summary__c. We only need to update Balance__c,  the catch is if there is more that one Advance_Summary__c records related to an Account, we Only update the most recently created.

Below is the code.
 

/* Json string im getting looks like this.
[
	{
	"IDCUST": "RC-002394", //Mapped to RC_Account_No__c on Account
	"AMTBALDUEH": 1345.89 //Mapped to Balance__c on Advance_Summary__c
	},
	{
	"IDCUST": "RC-002408",
	"AMTBALDUEH": 133.33
	}
]
*/

public class CustomerStatsBatch 
{   
    @future(callout=true) 
    public static void executeThis() {
       
        Http h = new Http();    
        HttpRequest req = new HttpRequest();    
		
		//Calling the endpoint URL
        req.setEndpoint('http://crm.retal.com/SageERPAPI/api/CustomerStats'); 
 
        req.setMethod('GET');
    
        HttpResponse res = h.send(req);
    
        String rNf =res.getbody();
        String jsonString = '{"items": '+res.getbody()+'}'; 
		/* Above I format the JsonString so it looks like below
		{
		"items": [
				{
				"IDCUST": "RC-002394",
				"AMTBALDUEH": 1345.89
				},
				{
				"IDCUST": "RC-002408",
				"AMTBALDUEH": 133.33
				}
			    ]
		}
	     */
		
		//Deserialize it using apex class JSON2ApexCustomerStats
        JSON2ApexCustomerStats objResponseBody = 
					(JSON2ApexCustomerStats)JSON.deserialize(jsonString, JSON2ApexCustomerStats.class);

        List <Account> updateAcc = new List<Account>{};
        integer count = 0;
        
        List<String> rcNums = new List<String>{};
        List<Account> list2bUpdated = new List<Account>{};
        List<Advance_Summary__c> plswork = new List<Advance_Summary__c>{};
        
        //Load RC Numbers you're getting from json string so I can use them as an index
        for(JSON2ApexCustomerStats.items row: objResponseBody.items) {
            rcNums.add(row.IDCUST);
        }  
       
	    //The first query selects an account with aleast 1 child, the most recent if there more than 1
        for(Account rec : [select Id, RC_Account_No__c,
							(select id, Balance__c from Account.Balance__cr order by createddate desc limit 1) 
							from Account where RC_Account_No__c in: rcNums and Id In 
							(Select Advance_Summary__c from Advance_Summary__c) ]){
            for(JSON2ApexCustomerStats.items row: objResponseBody.items) {
            
                if(row.IDCUST == rec.RC_Account_No__c){
                    Account l = new Account();
                    l.id = rec.id;                   
                    list2bUpdated.add(l);
                    
                    Advance_Summary__c advSum = new Advance_Summary__c();
                    
                    //error in the line below if null;                         
                    advSum.id= rec.Advance_summary__r[0].id;
                    
                    advSum.Balance__c = row.AMTBALDUEH;
                    plswork.add(advSum);
                }
            }            
        }

        Database.update(list2bUpdated,false); 
        
        Database.update(plswork,false); 
        
        system.debug('list2bUpdated:'+ list2bUpdated);        
        system.debug('plswork:'+ plswork);
       
      }    
}

Your assistance will be appreciated and indicated as the best answer.