How do I get an array of the property of a certain class from an array of that class?

You don't tell how you defined CKRecord, but I guess it contains at least

var iD: CKRecordID // Note that a property should start with lowercase

Did you look at using map ?

So, the code I tested is as follows:

typealias CKRecordID = String

struct CKRecord {
    var iD: CKRecordID // Note that a property should start with lowercase
}

let record1 = CKRecord(iD: "1")
let record2 = CKRecord(iD: "2")
let record3 = CKRecord(iD: "3")

let records: [CKRecord] = [record1, record2, record3]

let recordIDs: [CKRecordID] = records.map( { (record: CKRecord) -> CKRecordID in
    let id = record.iD
    return id
} )

print(recordIDs)

you can use a much more compact map closure:

let recordIDs: [CKRecordID] = records.map { record in record.iD }