func_Realtime in Asterisk 1.6.0.6

Hello,
i use asterisk 1.6.0.6 with res_ldap.conf
and i want to get the an attribut from ldap but when using func_realtime all attributes i get all attributes. Is it possible to use in like the older realtime application?

when using
exten => 250,2,Set(Var=${REALTIME(sip,AstAccountCallerID,{$EXTEN}})

i got
Var=objectClass=top,objectClass=person,objectClass=organizationalPerson,objectClass=inetOrgPerson,objectClass=posixAccount,objectClass=shadowAccount…

but i only want to get uid=