[3.9.5 but reproducible on latest] `DeepMerger` returns incorrect results when merging incremental arrays into existing data

[varunbezzam]

Issue Description

We have code that fetches arrays of data in the incremental responses using the @defer directive. When the previous data that is already cached is a superset of the data that is returned in the incremental response, the merging in of incremental data into existing data is incorrect. We expect the incremental data to replace the existing data entirely since it is missing some of the elements in the existing data, but this is not happening currently.

Link to Reproduction

Reproducible with a unit test as shown below:

Reproduction Steps

Please add:

it("should merge arrays of objects", function () {
    const a = [{ a: 1, b: 2 }, { c: 3, d: 4 }];
    const b = [{ a: 1, b: 2 }];
    const merger = new DeepMerger();
    expect(merger.merge(a, b)).toEqual([{ a: 1, b: 2 }]);
  });

into mergeDeep.ts tests since this reproduces our use case.

@apollo/client version

3.9.5

[triagster]

Related issues:

• Merging array in DeepMerger #11704

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[phryneas]

Hmm.
Looking at this, I don't think we need to change this in DeepMerger, but we'll probably have to revisit the behaviour introduced in #10334 - that merging there doesn't take typePolicies into account, so we might need to bring the cache into this logic more.

I'll bring this up at our next team meeting!

[varunbezzam]

@phryneas Thanks for your response! Yes, we are currently patching Apollo in QueryInfo.ts to have the initial response merged with the existing cached data except for any deferred fields. For deferred fields, we are currently removing their keys from the merged data to allow the incremental responses to fill out the data instead and avoid issues with merging data but as you said this won't work well with cache-first or cache-and-network policies.

[phryneas]

Hi, just reporting back.

We're currently in the final stretches of Apollo Client 4.0, so we won't be able to fix this in 3.x and probably also won't be able to release a fix immediately in 4.0.
That said, in 4.0 we will make this merging behaviour pluggable so people can bring different defer implementations.
That should allow you to temporarily work around this in userland, and we will try to completely fix this in a follow-up to the 4.0 release.

[jerelmiller]

Hey @varunbezzam 👋

This should be solved once we release #12923 as part of 4.1 with @stream support. If you'd like to try it now, I'd love any feedback you have. We'll try and get an alpha out within the next week or two, but until then you can try out the PR build.

npm i https://pkg.pr.new/apollographql/apollo-client/@apollo/client@12923

Thanks for reporting!

[jerelmiller]

We've just released the first preview of @stream in 4.1.0-alpha.0 which includes the changes to better handle merging incremental arrays. I'd be grateful if you could try it out and provide feedback.

npm i @apollo/client@next

Thanks so much!