How to loop over child objects of master object in batch object

Question

I have a batch process to update Accounts with address info from the Cust_Address__c object. I use a Junc_Account_Address__c object as a junction. How do I loop through the accounts to update the addresses. Currently I get the error Loop must iterate over collection:Account

global class AddressManager implements 
    Database.Batchable<sObject>, Database.Stateful {

    // instance member to retain state across transactions
    global Integer recordsProcessed = 0;
    global Database.QueryLocator start(Database.BatchableContext bc) {
        return Database.getQueryLocator(
            'SELECT Id, Start_Date__c, End_Date__c, Account__c, Cust_Address__c, Recordtype' +
            'FROM Junc_Account_Address__c' + 
            'WHERE Start_Date__c <= TODAY AND End_Date__c >= TODAY'  
        );
    }
    global void execute(Database.BatchableContext bc, List<Junc_Account_Address__c> scope){
        // process each batch of records
        List<Account> accountsToUpdate = new List<Account>();
        for (Junc_Account_Address__c ac : scope) {
            for (Account acc : ac.Account__r) {
                acc.BillingStreet = ac.Cust_Address__r.BillingStreet__c;
                acc.BillingPostalCode = ac.Cust_Address__r.BillingPostalCode__c;
                acc.BillingCity = ac.Cust_Address__r.BillingCity__c;
                acc.BillingState = ac.Cust_Address__r.BillingState__c;
                // add account to list to be updated
                accountsToUpdate.add(acc);
                // increment the instance member counter
                recordsProcessed = recordsProcessed + 1;
            }
        }
        update accountsToUpdate;
    }    
    global void finish(Database.BatchableContext bc){
        System.debug(recordsProcessed + ' records processed. Shazam!');
        AsyncApexJob job = [SELECT Id, Status, NumberOfErrors, 
            JobItemsProcessed,
            TotalJobItems, CreatedBy.Email
            FROM AsyncApexJob
            WHERE Id = :bc.getJobId()];
        // call some utility
    }    
}

Answer 1

Use __r child relationship query if in your Account object actually has a lookup field to Junc_Account_Address__c. The inner for loop below is not needed.

    for (Junc_Account_Address__c ac : scope) {
        for (Account acc : ac.Account__r) {
        }
    }

Also, since you are referencing some fields from the Cust_Address__c, you need to include them in your SOQL string. Otherwise you'll get the exception

"SObject row was retrieved via SOQL without querying the requested field"

    SELECT Id, Start_Date__c, End_Date__c, Account__c, 
        Cust_Address__r.BillingStreet__c, 
        Cust_Address__r.BillingPostalCode__c, 
        Cust_Address__r.BillingCity__c, 
        Cust_Address__r.BillingState__c, 
        Recordtype
    FROM Junc_Account_Address__c
    WHERE Start_Date__c <= TODAY AND End_Date__c >= TODAY

    global void execute(Database.BatchableContext bc, List<Junc_Account_Address__c> scope){
        // process each batch of records
        List<Account> accountsToUpdate = new List<Account>();
        for (Junc_Account_Address__c ac : scope) {
            Account acc = new Account (
                Id = ac.Account__c,
                BillingStreet = ac.Cust_Address__r.BillingStreet__c,
                BillingPostalCode = ac.Cust_Address__r.BillingPostalCode__c,
                BillingCity = ac.Cust_Address__r.BillingCity__c,
                BillingState = ac.Cust_Address__r.BillingState__c
            );
            // add account to list to be updated
            accountsToUpdate.add(acc);
            // increment the instance member counter
            recordsProcessed = recordsProcessed + 1;
        }
        update accountsToUpdate;
    }