// ❌ NEVER — causes LimitException at scale
for (Account a : accounts) {
    List<Contact> contacts = [SELECT Id FROM Contact WHERE AccountId = :a.Id]; // SOQL in loop
    update a; // DML in loop
}

// ✅ ALWAYS — collect, then query/update once
Set<Id> accountIds = new Map<Id, Account>(accounts).keySet();
Map<Id, List<Contact>> contactsByAccount = new Map<Id, List<Contact>>();
for (Contact c : [SELECT Id, AccountId FROM Contact WHERE AccountId IN :accountIds]) {
    if (!contactsByAccount.containsKey(c.AccountId)) {
        contactsByAccount.put(c.AccountId, new List<Contact>());
    }
    contactsByAccount.get(c.AccountId).add(c);
}
update accounts; // DML once, outside the loop

// Check before querying a field
if (!Schema.sObjectType.Contact.fields.Email.isAccessible()) {
    throw new System.NoAccessException();
}

// Or use WITH USER_MODE in SOQL (API 56.0+)
List<Contact> contacts = [SELECT Id, Email FROM Contact WHERE AccountId = :accId WITH USER_MODE];

// Or use Database.query with AccessLevel
List<Contact> contacts = Database.query('SELECT Id, Email FROM Contact', AccessLevel.USER_MODE);

// ❌ NEVER — concatenates user input into SOQL string
String soql = 'SELECT Id FROM Account WHERE Name = \'' + userInput + '\'';

// ✅ ALWAYS — bind variable
String soql = [SELECT Id FROM Account WHERE Name = :userInput];

// ✅ For dynamic SOQL with user-controlled field names — validate against a whitelist
Set<String> allowedFields = new Set<String>{'Name', 'Industry', 'AnnualRevenue'};
if (!allowedFields.contains(userInput)) {
    throw new IllegalArgumentException('Field not permitted: ' + userInput);
}

@isTest(SeeAllData=false)   // Required — no exceptions without a documented reason
private class AccountServiceTest {

    @TestSetup
    static void makeData() {
        // Create all test data here — use a factory if one exists in the project
    }

    @isTest
    static void givenValidInput_whenProcessAccounts_thenFieldsUpdated() {
        // Positive path
        List<Account> accounts = [SELECT Id FROM Account LIMIT 10];
        Test.startTest();
        AccountService.processAccounts(accounts);
        Test.stopTest();
        // Assert meaningful outcomes — not just no exception
        List<Account> updated = [SELECT Status__c FROM Account WHERE Id IN :accounts];
        Assert.areEqual('Processed', updated[0].Status__c, 'Status should be Processed');
    }
}