Try the following:

openvpms:get(., 'patient.objectReference')/id

openvpms:get(., 'customer.objectReference')/id

Neither of these calls are very efficient as the relevent objects are retrieved first in order to determine their ids.

Performance can be improved by exposing the PropertySet used to cache the properties as a Map.

You could then access the properties via jxpath something like

properties/patient.objectReference/id

properties/customer.objectReference/id

without the requirement to retrieve the  corresponding patient/customer objects.

Also note that neither of the above properties are visible to the 'Edit Display Expression' editor.

You'll get an 'Expression Error' message if you click 'Test'.

All of the properties it knows about are listed in the 'Expression Properties' box.

The full set of available properties are:

act.objectReference

act.startTime

act.endTime

act.status

act.statusName

act.reason

act.reasonName

act.description

schedule.objectReference

schedule.name

customer.objectReference

customer.name

patient.objectReference

patient.name

scheduleType.objectReference

scheduleType.name

clinician.objectReference

clinician.name

arrivalTime

If you want to access customer and patient properties other than the above, you need to consult their corresponding archetype descriptors.

You'll need to access their properties using the references as described above.

E.g to get a patient breed you can do:

openvpms:lookup(openvpms:get(., 'patient.objectReference'), 'breed')