Associations should be able to alias a different property

[epixa]

The current behavior of associations is to rely on the data in a property on the model by the same name, so:

comment.hasOne('article');
comment.article() // proxies using the value of the `article` property

This should remain the default behavior, but you should be able to optionally define a proxy as an alias for a different value, for example:

comment.hasOne('article', { alias_of: 'article_url' });
comment.article() // proxies using the value of the `article_url` property

One important thing to consider: when an alias is set up in this way, it's important that we can still define article_url as an attribute but do not have to define it as an attribute.