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Map key {some key} not found in map
I need to map the account.name from a contact record to a custom field on the user object when I enable that contact as a portal user. Since this conversion brings a standard user page, is there anywhere I can set my value without using a trigger?
Not AFAIK but you could also create a simple Visualforce page to do this. Add a button you can click on from the Contact record and voila you are there.
It is not possible without trigger because you want to update User object's field by updated Contact object's field.