gistfile1.js

YUI().use('model', 'model-list', function (Y) {

    Y.Tag = Y.Base.create('tag', Y.Model, [], {}, {
        ATTRS: {
            name: {}
        }
    });

    Y.Tags = Y.Base.create('tags', Y.ModelList, [], { model: Y.Tag });

    Y.Post = Y.Base.create('post', Y.Model, [], {
        _setTags: function (tags) {
            // Make sure we just have a native Array of Tag instances.
            tags = Y.Array(tags instanceof Y.ModelList ? tags.toArray() : tags);
            // Update the Tags instance with the new tags.
            this.get('tags').reset(tags);
        }
    }, {
        ATTRS: {
            title: {},
            body : {},
            tags : {
                value : new Y.Tags,
                setter: '_setTags'
            }
        }
    });

    var post = new Y.Post({
        title: 'Foo Post',
        body : 'Bla bla bla…',
        tags : [
            { name: 'YUI' },
            { name: 'App Framework' }
        ]
    });

    post.get('tags').add({ name: 'JavaScript' });

});