Hi dr_death,

I personally use Moo rather than Moose as I find the latter has lots of power tools I never need. However, I believe the answer is mostly the same for the two systems. You probably want to use default or builder as described at http://search.cpan.org/~ether/Moose-2.2004/lib/Moose/Manual/Attributes.pod#Default_and_builder_methods. Since you'll presumably need one of the other attributes to use in the DB query, you'll probably have to make the attribute lazy and use builder. Something like:

has 'email' => (
    is        => 'rw',
    isa       => 'Str',
    lazy      => 1,
    builder   => '_lookup_email',
);

sub _lookup_email {
    my $self = shift;
    return $self->_dbh->selectall_array(
        'select email from users where name = ?',
        {},
        $self->name,
    )->[0];
);
[download]

Obviously you'll have to supply the DB connection, error handling if the email is not found, etc. Also note that the Moo[se] Type libraries should be able to provide an email address type, so your isa could be email or similar and the value will be checked.

Edit: On re-reading I see that you maybe want to supply the email address and return the DB ID after adding a record? If so, just change the builder sub to do what you want!

Hope this helps!