Parse cloudcode beforeSave obtain pre-updated object

Question

In a beforeSave hook I want to obtain the state of the object prior to the update. In this particular case it is to stop a user from changing their choice once

Answer 1

The request variable is the updated row itself. You can get it's object id through request.object.idand use this to grab the current row from the database and check the current value, like so:

Parse.Cloud.beforeSave('votes', function(request, response) {
    if (!request.object.isNew()) {
    var query = new Parse.Query("votes");
    query.get(request.object.id, { // Gets row you're trying to update
        success: function(row) {
            if (row.get('choice') !== null) 
                response.error('Not allowed to change your choice once submitted');
            response.success(); // Only after we check for error do we call success
        },
        error: function(row, error) {
            response.error(error.message);
        }
    });
}