Indexable Relations

Datastore structure:

```
Author:12345:books:9876 → 9876  // normal
Author:books:9876 → 12345  // isIndexable
```

Difficulty in asserting that each relation is unique (esp. with OneToMany Relationships).

Possibly could use a AuthorInstance → OneToOne → CollectionInstance → OneToMany → BookInstances, and make a unique CollectionInstance?