How to Avoid Mixed DML Error in Salesforce Test Classes

Understanding the Mixed DML Error

The Mixed DML error occurs when performing DML operations on both setup objects (like User, Profile, Permission Set) and non-setup objects (like Account, Contact) in the same transaction. This restriction exists because setup objects affect user access within Salesforce.

Causes of the Mixed DML Error

1. Creating or updating both a User object and standard objects (like Account, Contact) in the same test method.

2. Modifying setup objects like UserRole, Profile, or PermissionSet alongside standard objects.

3. Using automation that affects both types of objects in a single transaction.

4. Working with custom settings (which are setup objects) and standard records in the same execution context.

How to Handle the Mixed DML Error

Method 1: Use System.runAs() Blocks

apex
@isTest
static void testMethod1() {
    // Create standard objects first
    Account acc = new Account(Name = 'Test Account');
    insert acc;
    
    // Then use runAs for setup objects
    System.runAs(new User(Id = UserInfo.getUserId())) {
        Profile p = [SELECT Id FROM Profile WHERE Name = 'Standard User'];
        User testUser = new User(/* User fields here */);
        insert testUser;
    }
}

Method 2: Use @future Methods

apex
@future
public static void createTestUser() {
    Profile p = [SELECT Id FROM Profile WHERE Name = 'Standard User'];
    User testUser = new User(/* User fields here */);
    insert testUser;
}

@isTest
static void testMethod1() {
    // Create standard objects
    Account acc = new Account(Name = 'Test Account');
    insert acc;
    
    // Call the future method
    Test.startTest();
    createTestUser();
    Test.stopTest(); // Ensures future method executes
}

Method 3: Separate Transactions with Test.startTest() and Test.stopTest()

apex
@isTest
static void testMethod1() {
    // Setup objects first
    Profile p = [SELECT Id FROM Profile WHERE Name = 'Standard User'];
    User testUser = new User(/* User fields here */);
    insert testUser;
    
    Test.startTest();
    // Non-setup objects in a separate transaction
    Account acc = new Account(Name = 'Test Account');
    insert acc;
    Test.stopTest();
}

Conclusion

Mixed DML errors can be frustrating but are easily resolved with proper test class design. The most reliable approaches are using System.runAs() blocks, @future methods with Test.startTest()/Test.stopTest(), or organizing your code to handle setup and non-setup objects in separate transactions.

Remember that the goal is to separate operations on setup objects from operations on non-setup objects to maintain data integrity and proper access permissions within Salesforce.