setWhatId in salesforce using email template - salesforce

Question

i have to send an email to a user in salesforce using email template.this template contain merge field type of custom object.
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
mail.setTargetObjectId(user.get(0).id);
mail.setTargetObjectId(user.get(0).Id)
mail.setTemplateId(specifier.get(0).Template_id__c);
mail.saveAsActivity = false;
mail.setWhatId(custom_object.Id);
i read in documentation
If you specify a contact for the targetObjectId field, you can specify a whatId as well. This helps to further ensure that merge fields in the template contain the correct data. The value must be one of the following types:
Account
Asset
Campaign
Case
Contract
Opportunity
Order
Product
Solution
Custom
but if we are sending email to a user not to contact then how to assign a custom object for merge field type in custom objects as in the above code

Answer 1

This is a GIGANTIC whole in their email methods, and one that has annoyed me for years. Particularly given workflow email alerts seem to have no problem sending an email template for a user. Alas, you can't use setWhatId() if your target is a user. But you can vote for them to add that functionality,
I've worked around this I typically create a contact with the same name and email as the user, use it to send the email, and then delete it. This works well, although dealing with validation rules on the contact object can be a challenge. See their dev boards for a full discussion.

Answer 2

You can get the template and replace the merge fields as follows:
EmailTemplate template = [SELECT Id, Subject, HtmlValue, Body FROM EmailTemplate WHERE Name = 'Case Update'];
Case modifiedCase = [SELECT Account.Id, Account.Name, Owner.FirstName, Owner.LastName, CaseNumber, Subject, LastModifiedBy.FirstName, LastModifiedBy.LastName from Case where Id=:modifiedCaseId];
String subject = template.Subject;
subject = subject.replace('{!Case.Account}', modifiedCase.Account.Name);
subject = subject.replace('{!Case.CaseNumber}', modifiedCase.CaseNumber);
subject = subject.replace('{!Case.Subject}', modifiedCase.Subject);
String htmlBody = template.HtmlValue;
htmlBody = htmlBody.replace('{!Case.Account}', modifiedCase.Account.Name);
htmlBody = htmlBody.replace('{!Case.OwnerFullName}', ownerFullName);
...
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
email.setSubject(subject);
email.setHtmlBody(htmlBody);
Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
As far as no new fields are added in the template this will work fine. The admin can mess with the format of the email without the need for code changes.

Answer 3

Not sure this is possible to do, but it depends on the relationship between your custom object and your users that will be receiving the merged emails. Do you have a 1-to-1 relationship between User and CustomObject? If so, perhaps adding a reference to the single custom object instance that each user object references and then adding custom formula fields on your user object with CustomObject__r.CustomField__c would do the trick.
In a custom formula field on your User object:
TEXT(CustomObject__r.CustomField__c)
Then your template could be changed into a User template and the merge fields would be the formula fields that actually pointed to your custom object instance. But if you have some other relationship like 1-to-many or many-to-many between User and CustomObject__c, I think you're out of luck.