SUGGESTION for v5: Populate should avoid mutating properties

IMHO, populate mutating an _id into a document is wrong. Mutations make building CRUD services more difficult than it should be because the object populated cannot be sent back with modifications without further processing to restore the id.

If possible, I would like to see an `alias` in v5 so that instead of populating the id, it would populate a new property named as defined in the `alias` as shown herebelow:

```
var storySchema = Schema({
authorId: { type: Schema.Types.ObjectId, ref: 'Person', alias: 'author' },
title: String,
});
```

This would be the equivalent of AS in SQL Queries.

Without designating an `alias`, the behavior would remain the same as in v4 to stay compatible with previous versions.

Start by tracing the existing populate behavior and schema path handling in the Mongoose codebase; the issue does not identify specific files or tests. Compare the proposed alias behavior with current v4-compatible behavior, and consider the feature done when populated values use the configured alias without mutating the original _id semantics and existing behavior remains unchanged without an alias.

Written by the indexing model from the issue text.