How to avoid MIXED_DML_OPERATION error in Salesforce tests that create Users

Not many Salesforce people on here yet, I guess.

I found a solution, I don't know why it works, but it works.

All parts of the test that access normal objects need to be wrapped in a System.runAs that explicitly uses the current user, like this:

User thisUser = [ select Id from User where Id = :UserInfo.getUserId() ];
System.runAs ( thisUser ) {
    // put test setup code in here
}

So, the example text_mixed_dmlbug method given in the question, would become:

static testMethod void test_mixed_dmlbug() {  
    User u;
    Account a;      
    User thisUser = [ select Id from User where Id = :UserInfo.getUserId() ];
    System.runAs ( thisUser ) {
        Profile p = [select id from profile where name='(some profile)'];
        UserRole r = [Select id from userrole where name='(some role)'];
        u = new User(alias = 'standt', email='[email protected]', 
            emailencodingkey='UTF-8', lastname='Testing', 
            languagelocalekey='en_US', 
            localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
            timezonesidkey='America/Los_Angeles', 
            username='[email protected]');
        a = new Account(Firstname='Terry', Lastname='Testperson');
        insert a;
    }
    System.runAs(u) {
        a.PersonEmail = '[email protected]';
        update a;
    }

}

Then the MIXED_DML_OPERATION errors stop happening.

This behavior is actually documented in the salesforce documentation: http://www.salesforce.com/us/developer/docs/apexcode/index_Left.htm#StartTopic=Content/apex_dml_non_mix_sobjects.htm?SearchType. Read where it say "Important The primary exception to this is when you are using the runAs method in a test"

It seems like you've found a workaround. I just wanted to try and clear up why you where getting this error.

I think you are running into this issue (per http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_dml_non_mix_sobjects.htm):

sObjects That Cannot Be Used Together in DML Operations

Some sObjects require that you perform DML operations on only one type per transaction. For example, you cannot insert an account, then insert a user or a group member in a single transaction. The following sObjects cannot be used together in a transaction:

* Group1
* GroupMember
* QueueSObject
* User2
* UserRole
* UserTerritory
* Territory

Important The primary exception to this is when you are using the runAs method in a test.

In addition, the Summer 08 Release notes (that link is a PDF) say:

In previous releases, in a single transaction that involved triggers, you could perform DML operations on more than one type of sObject, for example, you could insert an account, then insert a user. As of Summer '08, you can only perform DML operations on a single type of sObject from the following list of sObjects.

For example, you cannot insert an account, then insert a user, or update a group, then insert a group member.

• Group
• GroupMember
• QueueSObject
• User
• UserRole
• UserTerritory
• Territory

In addition, User and Territory now support the insert and update DML operations, and UserRole now supports the insert, update delete and upsert DML operations.

Apex DML operations are not supported on the following sObjects:

• AccountTerritoryAssignmentRule
• AccountTerritoryAssignmentRuleItem
• UserAccountTeamMember