Query returns objects with different identities even though data hasn't changed

[jsslai]

I have a query which returns array of objects. On the UI I'm skipping rendering if single object has same identity oldObject === newObject. Now ApolloClient seems to always return objects with different identities.

Intended outcome:
Query returns objects with same identities because nothing changed.

Actual outcome:
Objects have different identities.

How to reproduce the issue:
Small repro where objects are compared to previous result: https://codesandbox.io/s/4w73lqw097

Versions
apollo-client: ^2.4.3 => 2.4.3
apollo-cache-inmemory: ^1.3.6 => 1.3.6
apollo-link-state: ^0.4.2 => 0.4.2

[benjamn]

This is likely a combination of e66027c (which clones the lastResult to protect against mutation) and the fact that we still pass that previousResult object into InMemoryCache#diff when broadcasting watches here. This logic was updated recently in d6a673f because the === check was not sufficient to establish that nothing has changed, but I can understand why it gets in the way of your design. In all likelihood, the newData.result object is === to your previous result, but the previousResult (which is only isEqual to newData.result) overrides it.

I believe we should be able to remove the previousResult logic altogether, but the last time we tried that some folks reported missing broadcasts. A lot has changed since then, so maybe it's time to try again… or possibly make this behavior configurable/opt-in.

[benjamn]

@jsslai I can confirm that #4069 will fix your reproduction, though it seems to have broken some tests that expect the query observable next method to be fired a certain number of times. Not sure exactly what changed there, but I will figure it out.

[jsslai]

Thanks! I think many React apps do shallow equality checks and this has affect on performance. Missing broadcasts is a problem but I can't think of a use case where app needs to get a broadcast when nothing changed (data, status or error).

[benjamn]

This should be fixed by apollo-client@2.4.5!

[jsslai]

It works now. :)