Impersonate another user when constructing `Results`

Goals

Now that sync has implemented an API to impersonate another user it would be great to get this API exposed through Realm JS.

Note: This feature is probably only relevant for an admin user which has opened a reference Realm.

I believe this should be exposed on the Results class, but perhaps it would be possible to add it on the List too and we could put the method on the Collection instead? I’m imagining an API where a new method is added to the Results class to get the results filtered by the permissions of another user:

---

1st API suggested

interface Results<T> extends Collection<T> {
    /**
     * Query as a specific user, by applying fine-grained permissions to the elements of the result.
     * @param {string} userId The ID of the user which we want to apply permissions for.
     * @returns Results The results filtered by applying the fine-grained permissions.
     */
    as(userId: string): Results<T>;
}

An example of usage could be

const realm = new Realm({ ... });
const wines = realm.objects<IWine>("Wine");
const oldMerlots = wines.filtered('variety == "Merlot" && vintage <= $0', 2010);

const johnsUserId = "0a8dfebc-40b8-11e9-b210-d663bd873d93";
const johnsOldMerlots = oldMerlots.as(johnsUserId);
console.log(`John has access to ${johnsOldMerlots.length} old merlots.`);

---

2nd API suggested (by @nirinchev 👇)

class Realm {
    /**
     * Create a user specific wrapper around the open Realm to perform queries as a specific user.
     * @param {string} userId The ID of the user which we want to apply permissions for.
     * @returns Realm A wrapper around the Realm instance which has modified behaviour to check fine-grained permissions when querying for and writing objects.
     */
    as(userId: string): Realm;
}

An example of usage could be

const realm = new Realm({ ... });
const johnsUserId = "0a8dfebc-40b8-11e9-b210-d663bd873d93";
const realmAsJohn = realm.as(johnsUserId);
const johnsWines = realmAsJohn.objects<IWine>("Wine");
const johnsOldMerlots = johnsWines.filtered('variety == "Merlot" && vintage <= $0', 2010);
console.log(`John has access to ${johnsOldMerlots.length} old merlots.`);

[nirinchev]

Yes, we need another change in sync to facilitate impersonated write transactions. I was thinking more of wrapping the Realm into a thin js wrapper that replaces some methods with impersonated ones, so the Realm would be opened once as an admin user and then, you would call const impersonate = realm.as('abc...') which will return an object with the same API as Realm but will replace .objects, .write, .objectForPrimaryKey and so on with user versions.

[kraenhansen]

I believe this can safely be closed now and perhaps reopened once we get more clarity on what replaces query-based sync and client-side permissions.