Topic: Parent app model associations and engines

Building an engine, but when it comes to associations with the parent app, I'm a little confused.

If I know a variable is going to be defined, like @user… and I want to be able to do something like

@user.user_assets 

instead of

@user_assets = UserAsset.for_user(@user)

what should I be doing in addition to has_many and belongs_to? Couldn't really see anything on Rails Guides.