How to subscribe to observable in observableArray

Question

I'm using KnockoutJS and trying to subscribe to an observable that is in an observableArray that is in an observableArray. So my viewModel looks like this...

function viewModel() {
    // private properties
    var self = this;

    // public properties
    self.movies = ko.mapping.fromJS([]);

    // subscriptions
    self.movies.UserMovies.Rating.subscribe(function(newValue) {
        console.log(newValue);
    });
}

The movies observableArray would look like this once populated from the mapping plugin...

[{
    Id: 1,
    Title: 'Movie1',
    Year: 2010,
    UserMovies: [{ Id: 11, Rating: 3.5, IsWatched: true }]
},{
    Id: 2,
    Title: 'Movie2',
    Year: 2010,
    UserMovies: [{ Id: 4, Rating: 4, IsWatched: true }]
}]

I'm trying to set up a subscription to UserMovies.Rating but, from my above viewModel getting the error message

TypeError: self.movies.UserMovies is undefined

How would I got about setting up a subscription to UserMovies.Rating when it is populated from the mapping plugin?

Answer 1

Knockout does not provide the granularity for knowing which items changed in an array, just that something changed. You will need to loop trough the array each time an item is added or removed.

The foreach binding (via ko.utils.compareArrays) actually calculates the minimum number of operations to transform one array into another one, so that DOM-elements does not need to be recreated.

Using ko.utils.compareArrays, I was able to create a method that subscribes to array changes at an item level. Leveraging this, I could write a select method that manages the subscriptions.

http://jsfiddle.net/MizardX/s9M4z/

With the new select method, you could do this pretty concisely:

// Subscribe to items added to the array. The returned 'subscription' will be
// disposed of, when the item is later removed.
viewModel.movies.select(function (movie) {

    // Return the subscription. Same as above.
    return movie.UserMovies.select(function (userMovie) {

        // Subscribe to a non-array. The callback will receive the updated value,
        // instead of the an added item.
        return userMovie.Rating.select(function (rating) {

            // Executed each time a rating is updated.
            console.log(movie.Id(), userMovie.Id(), rating);
        });
    });
});

It handles additions, updates, and deletions as expected.