APEX TRAINING -3:

This example shows how to perform mixed DML operations by using future method  to perform a DML operation on the user object
public class Util {
@future
public static void insertUserWithRole(
String uname, String al, String em, String lname) {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
// Create new user with a non-null user role ID
User u = new User(alias = al, email=em,
emailencodingkey='UTF-8', lastname=lname,
languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
timezonesidkey='America/Los_Angeles',
username=uname);
insert u;
}
}

public class MixedDMLFuture {

public static void useFutureMethod() {

// First DML operation

Account a = new Account(Name='Acme');

insert a;

// This next operation (insert a user with a role)

// can't be mixed with the previous insert unless

// it is within a future method.

// Call future method to insert a user with a role.

Util.insertUserWithRole(

'mruiz@awcomputing.com', 'mruiz',

'mruiz@awcomputing.com', 'Ruiz');

}

}

@isTest
private class MixedDML {
static testMethod void mixedDMLExample() {
User u;
Account a;
User thisUser = [SELECT Id FROM User WHERE Id = :UserInfo.getUserId()];
// Insert account as current user
System.runAs (thisUser) {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
u = new User(alias = 'jsmith', email='jsmith@acme.com',
emailencodingkey='UTF-8', lastname='Smith',
languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
timezonesidkey='America/Los_Angeles',
username='jsmith@acme.com');
insert u;
a = new Account(name='Acme');
insert a;
}
}
}