[Bug] Ember 3.20.4 Does not Update When ED related records are removed

[jrjohnson]

🐞 Describe the Bug

When records are removed from an ember data relationship the change is not being reflected in a template.

🔬 Minimal Reproduction

Reproduced in https://github.com/jrjohnson/reproduction-template-model-relationships
Steps:

1. npm start
2. Visit http://localhost:4200/tests?devmode
3. Open Console
4. Clicking Add POSTs a new record and re-computes the count and then {{log}} output.
5. Clicking Clear PATCHs and removes the records, but does not update the count of the {{log}}.

😕 Actual Behavior

Removing all records from an Ember Data relationship array does not update the template.

🤔 Expected Behavior

Changing this array should always change the template.

🌍 Environment

• Ember: 3.20.4
• Node.js/npm: v12.18.2 / 6.14.6
• OS: OSX
• Browser: FF/Safari/Chrome

[jrjohnson]

May be related to or a duplicate of #19101, but I'm not sure so I created this new issue with the reproduction.

[rwjblue]

@pzuraq this seems like a similar issue to emberjs/data#7273, would you mind taking a look to see what is going on? This is broken between 3.20.3 and 3.20.4.

[pzuraq]

Dug into this, and figured out the issue. Have a fix ready, just need to write a test, will be doing that in the morning

[jrjohnson]

Confirmed. This is fixed in v3.20.5. Thanks!

[jrjohnson]

@pzuraq I'm seeing a related issue when not depending on .length, but just watching the array. Something like

{{did-update this.load @post.comments}}

Should I open a new issue and reproduction? Or should that have been covered in this fix?