Accessing original field in Parse.com Cloud Code beforeSave function

Question

The ultimate goal is to detect changes between an existing Parse object and the incoming update using the beforeSave function in Cloud Code.

From the Cloud Code log available through parse.com, one can see the input to beforeSave contains a field called original and another one called update.

Cloud Code log:

Input: {"original": { ... }, "update":{...}

I wonder if, and how, we can access the original field in order to detect changing fields before saving.

Note that I've already tried several approaches for solving this without success:

using (object).changedAttributes()
using (object).previousAttributes()
fetching the existing object, before updating it with the new data

Note on request.object.changedAttributes(): returns false when using in beforeSave and afterSave -- see below for more details:

Log for before_save -- summarised for readability:

Input: { original: {units: '10'}, update: {units: '11'} }
Result: Update changed to { units: '11' }
[timestamp] false <--- console.log(request.object.changedAttributes())

Log for corresponding after_save:

[timestamp] false <--- console.log(request.object.changedAttributes())

object.changedAttributes() is the correct approach. Please post your attempt related to that, and we'll make it work. — danh
If you can produce a demonstration of that (changing an object, then finding changedAttributes() === false), then you'll have found a parse.com bug. I suspect, though, that you're mistaken. Please prove me wrong by posting a failure others can reproduce. — danh
Added details on the case I'm talking about. Note that I'm talking about Cloud Code, and not using the JS SDK on the client side. — marco alves
My apologies. I assumed you had a bug and just blamed parse. I decided, arrogantly, that if you were too lazy to build an MRE, I would do it and show you the error in your ways. Imagine my surprise to find the same issue! Sorry again. As penance, I built a work-around and posted it below. — danh

danh danh · Accepted Answer · 2015-06-30T20:37:28

There is a problem with changedAttributes(). It seems to answer false all the time -- or at least in beforeSave, where it would reasonably be needed. (See here, as well as other similar posts)

Here's a general purpose work-around to do what changedAttributes ought to do.

// use underscore for _.map() since its great to have underscore anyway
// or use JS map if you prefer...

var _ = require('underscore');

function changesOn(object, klass) {
    var query = new Parse.Query(klass);
    return query.get(object.id).then(function(savedObject) {
        return _.map(object.dirtyKeys(), function(key) {
            return { oldValue: savedObject.get(key), newValue: object.get(key) }
        });
    });
}

// my mre beforeSave looks like this
Parse.Cloud.beforeSave("Dummy", function(request, response) {
    var object = request.object;
    var changedAttributes = object.changedAttributes();
    console.log("changed attributes = " + JSON.stringify(changedAttributes));  // null indeed!

    changesOn(object, "Dummy").then(function(changes) {
        console.log("DIY changed attributes = " + JSON.stringify(changes));
        response.success();
    }, function(error) {
        response.error(error);
    });
});

When I change someAttribute (a number column on a Dummy instance) from 32 to 1222 via client code or data browser, the log shows this:

I2015-06-30T20:22:39.886Z]changed attributes = false

I2015-06-30T20:22:39.988Z]DIY changed attributes = [{"oldValue":32,"newValue":1222}]

Accessing original field in Parse.com Cloud Code beforeSave function

1 Answers