You need to change post: ['2']
to post: '2'
(since it's a belongsTo
relationship it expects a value not an array):
App.Comment = DS.Model.extend({
post: DS.belongsTo('post', { async: true }),
date_posted: DS.attr('date'),
author: DS.attr('string'),
message: DS.attr('string')
});
App.Comment.FIXTURES = [{
id: '1',
post: '2',
date_posted: new Date(),
author: 'Aaron',
message: 'Psyched for the third.'
}];