Support grandchildren in self-referrential models

The goal is to support:

```coffeescript
links:
  'sections.children': '/sections/{sections.id}/children'
sections = [
  id: 1
  name: 'Home'
  parent_id: null
  links:
    children: [2, 3]
,
  id: 2
  name: 'Child 1'
  links:
    children: [4, 5]
,
  id: 3
  name: 'Child 2'
,
  id: 4
  name: 'Grandchild 1'
,
  id: 5
  name: 'Grandchild 2'
]
```