addr := fromNetworkAddress(netAddr)                                     
        fieldName := "preferredprivateaddress"                                  
        current := m.doc.PreferredPrivateAddress                                
        if isPublic {                                                           
                fieldName = "preferredpublicaddress"                            
                current = m.doc.PreferredPublicAddress                          
        }                                                                       
                                                                                
        ops := []txn.Op{{                                                       
                C:      machinesC,                                              
                Id:     m.doc.DocID,                                            
                Update: bson.D{{"$set", bson.D{{fieldName, addr}}}},            
                Assert: bson.D{{fieldName, current}},                           
        }}