nestedFutures.swift

// assuming a route such as /parents/Parent.parameter/children/

func addChildtoParent(_ req: Request) throws -> Future<Child> {
  return try flatMap(
    to: Child.self, // what we will get back
    req.parameters.next(Parent.self), // an already existing Parameter conforming instance
    req.content.decode(Child.self)) { // a Content conforming instance
      parent, child in
    
        child.parentId = try parent.requireId()
        child.save(on: req)                                                                                 
}