Custom Identifier - how does it work?

Hello,

I am using Metabase for a while, and I am happy with it.
Now I am in a situation, that I have to use a custom identifier. The
scenario is that I use Metabase to synchronize data between existing
db/php app and a newly created web application. The existing app
cannot be changed, and a table containing user information has a
login filed as the primary key. An id field cannot be added.

Now my question: How can I get Metabase to work with a field login
insted of id?

Here is my config for that class, that still uses id as the primary
key:
<class>
  <id>login</id>
  <name>z_user</name>
  <variable>
    <name>login</name>
    <length>10</length>
    <type>text</type>
  </variable>
  <variable>
    <name>passwort</name>
    <type>text</type>
    <length>255</length>
  </variable>
  <variable>
    <name>email</name>
    <type>text</type>
    <length>50</length>
    <optional>1</optional>
  </variable>
</class>

Regards,
Manfred Schreck