[web2py] one to many relationship but Foreign key as non-id,unique field.

[Phyo Arkar]

I am Thinking to link two tables ,
"emails" and "attachments"

emails have a unique key , EmailID , which is a md5sum of the path to email
file

I am trying to link with another table which is attachment table , it will
have a field  'attachment_of'   which is link to "EmailID"


To Short

How can i link between primary and foreign keys which is  a
non-id_but_unique field?