Is there an easy way to get all the records of a model at once in a serverside script, or at least all keys ?
To get all records:
var allRecords = app.models.MODELNAME.newQuery().run();
To get all records keys:
var allRecordsKeys = app.models.MODELNAME.newQuery().run().map(function(item){return item._key;});
Reference: https://developers.google.com/appmaker/scripting/server#querying_records