Error: MIXED_DML_OPERATION Very Confused

Question

I'm very confused about MIXED_DML_OPERATIONerror. Following test case fails in production but passes in uat.

@IsTest
private class MyControllerTest {
    private static final String USERNAME ='[email protected]';

    @IsTest
    static void test() {
        ...
    }

    @TestSetup
    static void createMockObjects() {
        Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator'];
        User u = new User(
                Alias = 'tesAdmin',
                Email = '[email protected]',
                EmailEncodingKey = 'UTF-8',
                LastName = 'Test',
                LanguageLocaleKey = 'en_US',
                LocaleSidKey = 'en_US',
                TimeZoneSidKey='Australia/Sydney',
                ProfileId = p.Id,
                UserName = USERNAME
        );

        insert u;

        String customObjectString = '{"attributes":{"type":"Custom_Object__c"},"Name":"OBJ-01", ...}';
        Custom_Object__c customObj = (Custom_Object__c)JSON.deserialize(customObjectString, Custom_Object__c.class);
        insert customObj;
    }
}

Above test class runs perfect in UAT environment but fails in production with following error.

MIXED_DML_OPERATION, DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): Custom_Object__c, original object: S2XUserMap: []

In many forums it is said that cannot perform DML Operations for setup objects and non-setup objects in the same context. If that is the case, why this runs properly in UAT?

Note: This happens only after production org is migrated to a different server.

Answer 1

I'm not sure why this runs properly in UAT. But you can fix it by using System.runAs()

@TestSetup
    static void createMockObjects() {
        Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator'];
        User u = new User(
                Alias = 'tesAdmin',
                Email = '[email protected]',
                EmailEncodingKey = 'UTF-8',
                LastName = 'Test',
                LanguageLocaleKey = 'en_US',
                LocaleSidKey = 'en_US',
                TimeZoneSidKey='Australia/Sydney',
                ProfileId = p.Id,
                UserName = USERNAME
        );

        system.runAs(u){
            String customObjectString = '{"attributes":{"type":"Custom_Object__c"},"Name":"OBJ-01", ...}';
            Custom_Object__c customObj = (Custom_Object__c)JSON.deserialize(customObjectString, Custom_Object__c.class);
            insert customObj;
        }
        
    }