System-wide map function (to convert an id or array of ids to the appropriate model instance from model stores). Also demonstrates polymorphic associations.

map_polymorphic_assoc.js

var Fruit = Backbone.AssociatedModel.extend();
var Banana = Fruit.extend();
var Tomato = Fruit.extend();

var fruitStore = {};

fruitStore['Banana'] = [
    new Banana({species:"Robusta", id:3}),
    new Banana({species:"Yallaki", id:4}),
    new Banana({species:"Genetic Modification", id:5}),
    new Banana({species:"Organic", id:6})
];
fruitStore['Tomato'] = [
    new Tomato({species:"Cherry", id:3}),
    new Tomato({species:"Regular", id:4})
];

//Determine type at run-time
var polymorphic = function (relation, attributes) {
    var key = relation.key + '_type';
    return attributes[key] || this.get(key);

};

//Handles both an array of ids and an id. Converts and id (or array of ids) to instances from the fruit store
//System wide function. Can handle different fruit types
var map = function (fids, type) {
    fids = _.isArray(fids) ? fids : [fids];
    type = type instanceof Backbone.Collection ? type.model : type;
    var store = function (type) {
        if (type == Banana)
            return fruitStore['Banana'];
        if (type == Tomato)
            return fruitStore['Tomato'];
    }(type);

    return _.map(
        fids,
        function (fid) {
            return _.findWhere(store, {id:fid});
        }
    );
};

var Oatmeal = Backbone.AssociatedModel.extend({
    relations:[
        {
            type:Backbone.One,
            key:'fruitable',
            relatedModel:polymorphic,
            map:map
        }
    ]
});

var aHealthyBowl = new Oatmeal({fruitable_type:Banana, fruitable:{species:"Robusta"}});

//aHealthyBowl.get('fruitable') instanceof Banana == true
//aHealthyBowl.get('fruitable') instanceof Tomato == false


var aHealthyBowl2 = new Oatmeal({fruitable_type:Banana, fruitable:3});

//aHealthyBowl2.get('fruitable') instanceof Banana == true


//Test with Backbone.Many
var FruitExplosion = Backbone.AssociatedModel.extend({
    relations:[
        {
            type:Backbone.Many,
            key:'fruitable',
            relatedModel:polymorphic,
            map:map
        }
    ]
});

var bananaExplosion = new FruitExplosion({fruitable_type:Banana, fruitable:[3, 4]});

//bananaExplosion.get('fruitable').at(0).get('species') === "Robusta"
//bananaExplosion.get('fruitable').at(1).get('species') === "Yallaki"

bananaExplosion.get('fruitable').add([5, 6]);

//bananaExplosion.get('fruitable').at(2).get('species') === "Genetic Modification" 
//bananaExplosion.get('fruitable').at(3).get('species') === "Organic"


var tomatoExplosion = new FruitExplosion({fruitable_type:Tomato, fruitable:[3, 4]});

//tomatoExplosion.get('fruitable').at(0).get('species') === "Cherry"
//tomatoExplosion.get('fruitable').at(1).get('species') === "Regular"

Thanks to @monokrome for this example and motivation.