如何更新User.username？

Question

我有一个RESTful API，可让我更新某些内容，如下所示：

由于这是外部用户（使用个人帐户自动创建了一个用户）。我想知道为什么我不能更新用户名？如果我对此进行更新，则会出现错误：

  

10：38：10：978 USER_DEBUG [283] | DEBUG | ===用户更新错误===更新失败。 ID为0030m00000IztRaAAJ的第0行的第一个异常；第一个错误：MIXED_DML_OPERATION，在更新了非设置对象（反之亦然）之后，不允许对设置对象进行DML操作：反之亦然：帐户，原始对象：用户：[]

以下是代码：

public static CA_RSEUserResponse updateUserInfo(CA_RSEUserInfo rseUserInfo) {

    User u = null;
    CA_RSEUserResponse resp = new CA_RSEUserResponse();
    Savepoint sv = Database.setSavepoint();

    try {
        // 1. Update User
        u = [
                SELECT Username,Email,FirstName,LastName,Id,ContactId
                FROM User
                WHERE Id = :rseUserInfo.id
                LIMIT 1
        ];

        System.debug('=== User ID is ===' + rseUserInfo.id);
        System.debug('User Contact ID is ===' + u.ContactId);

        u.Username = rseUserInfo.email_address;
        u.Email = rseUserInfo.email_address;
        u.FirstName = rseUserInfo.first_name;
        u.LastName = rseUserInfo.last_name;
        update u;


        // 2. Find the related contact and fill in the info
        Contact foundContact = [
                SELECT LastName,FirstName,Birthdate,MailingCity,MailingState,
                        MailingPostalCode,Phone,CA_Subscription_Status__c,CA_Gender__c,
                        CA_Dependants__c,CA_SkillLevel__c,CA_FavoriteCuisines__c,
                        CA_RecipeDislikes__c,CA_RecipeLikes__c,CA_Retailer__c,
                        CA_LanguagePreference__c,CA_EnjoymentLevel__c,CA_RepeatFrequency__c,
                        CA_AdventureLevel__c,CA_CreatedProfile__c,CA_CompletedProfile__c,
                        CA_HouseholdAdults__c
                FROM Contact
                WHERE Id = :u.ContactId
                LIMIT 1
        ];

        System.debug('Birth Year ==='+rseUserInfo.birth_year);

        foundContact.LastName = rseUserInfo.last_name;
        foundContact.FirstName = rseUserInfo.first_name;
        foundContact.Birthdate = rseUserInfo.birth_year;
        foundContact.MailingCity = rseUserInfo.city;
        foundContact.MailingState = rseUserInfo.state;
        foundContact.MailingPostalCode = rseUserInfo.zip;
        foundContact.Phone = rseUserInfo.mobile_phone;
        foundContact.CA_Subscription_Status__c = rseUserInfo.subscription_status;
        foundContact.CA_Gender__c = rseUserInfo.gender;
        foundContact.CA_Dependants__c = rseUserInfo.dependants;
        foundContact.CA_SkillLevel__c = rseUserInfo.skill_level;
        foundContact.CA_FavoriteCuisines__c = rseUserInfo.favorite_cuisines;
        foundContact.CA_RecipeDislikes__c = rseUserInfo.recipe_dislikes;
        foundContact.CA_RecipeLikes__c = rseUserInfo.recipe_favorites;
        foundContact.CA_Retailer__c = rseUserInfo.retailer;
        foundContact.CA_LanguagePreference__c = rseUserInfo.language_preference;
        foundContact.CA_EnjoymentLevel__c = rseUserInfo.enjoyment_level;
        foundContact.CA_RepeatFrequency__c = rseUserInfo.repeat_frequency;
        foundContact.CA_AdventureLevel__c = rseUserInfo.adventure_level;
        foundContact.CA_CreatedProfile__c = rseUserInfo.created_profile;
        foundContact.CA_CompletedProfile__c = rseUserInfo.completed_profile;
        foundContact.CA_HouseholdAdults__c = rseUserInfo.household_adults;
        update foundContact;

        resp.errorMsg = null;
        resp.content = rseUserInfo;
    } catch (Exception ex) {
        Database.rollback(sv);
        System.debug('=== User Updated Error ===' + ex.getMessage());
        rseUserInfo = null;
    }

    return resp;

如果我不更新User.username，一切都很好……那为什么以及如何更新该字段？