rails/rails — pull request #7945 by Antiarchitect (2012-10-14 19:25:21+00:00)
Title: Query methods allowed for include option for collections.

Now you can do things like this:
respond_with @authors, include: [books: { order: "price ASC", limit: 5, where: ["price < ?", 5] }]
or just
@authors.to_xml(include: [books: { order: "price ASC", limit: 5, where: ["price < ?", 5] }])