[SalesForce] Send email with multiple related object information via email template

Create a dummy contact with the name Dummy and the email address you need. The code below should then allow you to get working from there

public void sendSingleEmailfromCustomSetting( string TemplateDeveloperName , string ToAddresses){
  EmailTemplate emailTemplate = [select Id, Body  from EmailTemplate where DeveloperName = :TemplateDeveloperName ];
  contact con=[Select id from contact Where FirstName = 'Dummy' limit 1];
  Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
  mail.setToAddresses( toAddresses.split( '[;,]' ) );
  mail.setSaveAsActivity(false);
  mail.setTargetObjectId(con.Id);
  mail.setSenderDisplayName(‘My Namej’);
  mail.setTemplateId(emailTemplate.id);
  mail.setWhatId(case.id);
  Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}

There is a request to enable cross-object formulas in email merge fields that has been there for 4 years, but apparently it's not on the roadmap here.

So yes, I can confirm, getting this data will require creation of a new formula field to hold this information as of April 2015.

Here's the relevant quote:

This request is not on our current roadmap, but we recognize it's value and the need for it.

We're currently exploring a related template revamp project (safe harbor) and if we move forward with that, we'll want to consider this request for development as part of that effort.