The rewrite of ember-data removed support for embedded records. The latest beta build adds support back, so you need to be using it.
First, the way you defined the embedded record is totally wrong. You need to define user attribute on articles as a relation to another model definition:
user: DS.belongsTo('user')
Then, you need to use a custom serializer with the Embedded Records Mixin to load embedded records for the model type:
I believe this functionality is in the Beta build only, so the documentation on emberjs.com won’t show it. If you read through the ember-data source code you will see documentation for embedded data.