@RoyH is 100% right to maintain your computed column as new objects are created. To do an initial migration, try a cloud function, like:
var _ = require("underscore");
Parse.Cloud.define("addFullnames", function(request, response) {
// useMasterKey if the calling user doesn't have permissions read or write to Table
Parse.Cloud.useMasterKey();
var query = new Parse.Query("Table");
// we'll call tables > 1000 an 'advanced topic'
query.limit = 1000;
query.find().then(function(results) {
_.each(results, function(result) {
var firstname = result.get("firstname") || "";
var lastname = result.get("lastname") || "";
result.set("fullname", (firstname + " " + lastname).trim());
});
return Parse.Object.saveAll(results);
}).then(function(results) {
response.success(results);
}, function(error) {
response.error(error);
});
});
Call it like this:
curl -X POST \
-H "X-Parse-Application-Id: your_app_id_here" \
-H "X-Parse-REST-API-Key: your_rest_key_here" \
-H "Content-Type: application/json" \
https://api.parse.com/1/functions/addFullnames
You can factor out the code inside _.each() to make a function that's called here and by a beforeSave hook to maintain the data as it is added.