setWhatId in salesforce using email template

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

Not exactly sure if this is possible as it depends on the relationships between your custom object and your users. In a custom formula field on your User object use the following code:

TEXT(CustomObject__r.CustomField__c)

Then your template can be changed into a User template and the merge fields would be the formula that actually pointed to your custom object instance.

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