How do I set a dynamic model attribute in the fixture?

As the title describes, I am running into trouble making a dynamic attribute on the Fixture layer.

Here is an example model:

App.Pokeball = DS.Model.extend({
    name: DS.attr('string'),
    ballRate: DS.attr('number'),
    battleAttributes: DS.belongsTo('battleAttributes')
});

And my Fixture:

App.Pokeball.reopenClass({
    FIXTURES : [
        {
            id: 1,
            name: 'PokeBall',
            ballRate: 1
        },
        {
            id: 23,
            name: 'Dusk Ball',
            ballRate: function() {
                // Some logic that applies only model #23
                return 2;
            }.property('battleAttributes')
        }
    ]
});

I scoured online trying to find out the right way to do this, but have instead ran into a dead end. :(