Do Salesforce VF email templates require related object to be persisted?

When a new lead comes in, I want to use a before trigger and a Visualforce email template that contains lead field values to send an email using the SingleEmailMessage class开发者_高级运维. The email is being generated, but all of the lead fields are null even though (known via System.Debug) they do have values going into the call.

Since I'm passing the still-unsaved lead Id via the mail.setWhatId(lead.Id) method, I'm beginning to think that the mail class is using the Id value and trying to do a database look-up rather than as a reference to the still unsaved lead in memory.

Does anyone know if that's the case? My class works flawlessly when the lead already exists.

If it is the case that the Apex mail class does a DB read, any pattern suggestions for the case where one needs to send and email and update a lead field value before the lead is saved? I can't use the Workflow email notification because the email is being addressed to customers, and there's some additional Apex code that sorts out what address to fetch from existing Account records based on some Lead fields--hence I think the need for using VF email templates in the first place.

setWhatId (and pretty much any method that takes an ID value as an argument) definitely does expect the row to be persisted already. To get around this, you should be able to just do your field update in the before trigger, and add an after trigger to send the email.