fix(netlify-cms-core): duplicate key warning

[barthc]

Summary
Fix the console warning on the workflow page

Warning: Encountered two children with the same key, .... Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.

that shows up when two entry from different collections both have the same slug. The issue shows for test-repo backend, not sure it would be the same case for github backend.

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit 0336614

https://deploy-preview-1930--netlify-cms-www.netlify.com

[verythorough]

Deploy preview for cms-demo ready!

Built with commit 0336614

https://deploy-preview-1930--cms-demo.netlify.com

[erquhart]

The index can change, so it isn't a reliable key. The slug should be unique, but it is possible for two entries from different collections to have the same slug. If you update to something like ${collection}-${slug}, that should work.

[barthc]

Yes I know the index key isn't reliable, but we are using it as keys in other places in the app 😊. So you want us to use ${collection}-${slug} in this case?

[erquhart]

We use the index in other places if we know the order of a list won't change, in which case it's safe to use. This isn't one of those cases, so we need to consider what will be ensured unique. I believe ${collection}-${slug} will work, but that's assuming your bug experience was due to two entries from different collections with the same slug. If your bug is reproducible and you can confirm that ${collection}-${slug} fixes it, works for me 👍

[barthc]

Updated the key to ${collection}-${slug}

[erquhart]

Thanks @barthc! Were you were able to confirm it addresses the bug you encountered?

[barthc]

@erquhart yes it did fix the bug.

[erquhart]

Awesome 👍👍