Many-to-One unique relationship

Nobody has claimed this yet.

I'm trying to get a cache working where the relationship is "many-to-one unique", but I think this package doesn't handle it. Here's an example:

I have a collection of Authors, and a collection of Comments. The authors store an array of `commentId`s, and I want each comment to denormalize the author, without it being an array.

```js
Authors // { commentIds: ['commentId1', 'commentId2'] }

// What I want
Comments: // { authorCache: { _id: 'authorId', name: 'John' } }

// What I get
Comments: // { authorCache: [{ _id: 'authorId', name: 'John' }] }
```

Usually many-to-one means there can be many authors linking to the same comment, but that's not the case here, so the cache is not correct.

The issue does not name a file or test entry point. Start by locating the relationship denormalization and cache-handling logic, then reproduce the Authors/Comments example. Done means the unique many-to-one relationship stores one author object rather than an array, with coverage for this case.

Written by the indexing model from the issue text.