func_Realtime in Asterisk

i use asterisk 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

but i only want to get uid=