KeyPathArray does not support computed properties like backlinks

[cmelchior]

Currently, notifications will trigger on a child object when parent objects in a backlink property are modified, but it isn't an option to control the behavior using the optional KeyPathArray.

The reason seems to be that the KeyPathArray is defined as this:

using KeyPath = std::vector<std::pair<TableKey, ColKey>>;
using KeyPathArray = std::vector<KeyPath>;

And since backlinks are computed properties they do not have a ColKey, so it isn't possible to build up a KeyPath that involves a backlink.

I'm not sure there is an easy fix as this implementation seems pretty central to the notification system, but I might have missed it. As a minimum I suspect the public API needs to change to:

using KeyPath = std::vector<std::pair<TableKey, Property>>;
using KeyPathArray = std::vector<KeyPath>;

No users have requested this feature yet as far as I know, I just ran into this limitation while adding support and tests for it to the Kotlin SDK. For now we will just throw an exception if a keypath involving backlinks are used.

[tgoyne]

Backlinks have column keys which you can obtain from Table::get_opposite_column() on the forward link. https://github.com/realm/realm-swift/blob/master/Realm/RLMClassInfo.mm#L114 and https://github.com/realm/realm-swift/blob/master/Realm/RLMClassInfo.mm#L79 are the relevant parts of Cocoa's implementation of string -> KeyPath that supports backlinks.

[cmelchior]

Ah cool. Closing this issue then as we will be adding that as part of #7087