Add test that shows a bug when merging deferred results for arrays. #11374

phryneas · 2023-11-20T13:46:44Z

This bug was brought to my attention by @fernando-apollo. We'll have to discuss how we can best fix this. I bet there are a few cases where this behaviour is really useful.

The bug showcased here is caused by this part of mergeResult:

apollo-client/src/core/QueryInfo.ts

Lines 375 to 383 in ae5091a

    
             // Detect the first chunk of a deferred query and merge it with existing 
        
             // cache data. This ensures a `cache-first` fetch policy that returns 
        
             // partial cache data or a `cache-and-network` fetch policy that already 
        
             // has full data in the cache does not complain when trying to merge the 
        
             // initial deferred server data with existing cache data. 
        
           } else if ("hasNext" in result && result.hasNext) { 
        
             const diff = this.getDiff(); 
        
             result.data = merger.merge(diff.result, result.data); 
        
           }

Checklist:

If this PR contains changes to the library itself (not necessary for e.g. docs updates), please include a changeset (see CONTRIBUTING.md)
If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
Make sure all of the significant new logic is covered by tests

that occurs with arrays in deferred queries

changeset-bot · 2023-11-20T13:46:48Z

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github-actions · 2023-11-20T13:47:58Z

size-limit report 📦

Path	Size
dist/apollo-client.min.cjs	37.08 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/main.cjs"	43.51 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/main.cjs" (production)	42.01 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/index.js"	32.61 KB (0%)
import { ApolloClient, InMemoryCache, HttpLink } from "dist/index.js" (production)	31.27 KB (0%)
import { ApolloProvider } from "dist/react/index.js"	1.22 KB (0%)
import { ApolloProvider } from "dist/react/index.js" (production)	1.21 KB (0%)
import { useQuery } from "dist/react/index.js"	4.28 KB (0%)
import { useQuery } from "dist/react/index.js" (production)	4.1 KB (0%)
import { useLazyQuery } from "dist/react/index.js"	4.59 KB (0%)
import { useLazyQuery } from "dist/react/index.js" (production)	4.41 KB (0%)
import { useMutation } from "dist/react/index.js"	2.55 KB (0%)
import { useMutation } from "dist/react/index.js" (production)	2.54 KB (0%)
import { useSubscription } from "dist/react/index.js"	2.24 KB (0%)
import { useSubscription } from "dist/react/index.js" (production)	2.2 KB (0%)
import { useSuspenseQuery } from "dist/react/index.js"	4.62 KB (0%)
import { useSuspenseQuery } from "dist/react/index.js" (production)	4.05 KB (0%)
import { useBackgroundQuery } from "dist/react/index.js"	4.14 KB (0%)
import { useBackgroundQuery } from "dist/react/index.js" (production)	3.56 KB (0%)
import { useReadQuery } from "dist/react/index.js"	3 KB (0%)
import { useReadQuery } from "dist/react/index.js" (production)	2.94 KB (0%)
import { useFragment } from "dist/react/index.js"	2.09 KB (0%)
import { useFragment } from "dist/react/index.js" (production)	2.04 KB (0%)

jerelmiller · 2024-07-05T22:01:28Z

Converting this to a draft since this contains a reproduction with a failing test.

uerbin · 2025-07-07T02:53:50Z

@phryneas Is this problem solved ?

phryneas · 2025-07-07T08:34:27Z

@uerbin It is not solved yet - this code path has been refactored heavily for Apollo Client 4.0 and we won't be able to fix this in time for 4.0, as this will need a lot of thorough testing.
That said, we are making preparations to fix this in a follow-up to 4.0:

With Apollo Client 4.0, the whole defer-merging solution will be abstracted away from the Apollo Client core, so different defer algorithms can be swapped out based on the server implementation. That will allow us to iterate on fixing this much better than would currently be possible.

jerelmiller · 2025-09-15T22:05:23Z

Superceded by #12923 which adds these tests to each incremental format.

add test that showcases a merge bug

e5136c4

that occurs with arrays in deferred queries

phryneas changed the title ~~Add test that shows a bug when merging deffered results for arrays.~~ Add test that shows a bug when merging deferred results for arrays. Nov 20, 2023

alessbell self-assigned this Nov 21, 2023

alessbell mentioned this pull request Mar 26, 2024

Merging array in DeepMerger #11704

Open

jerelmiller marked this pull request as draft July 5, 2024 22:01

phryneas mentioned this pull request Jan 6, 2025

Cache inconsistency with @defer and refetch #12250

Open

phryneas linked an issue Jan 6, 2025 that may be closed by this pull request

Cache inconsistency with @defer and refetch #12250

Open

jerelmiller closed this Sep 15, 2025

jerelmiller mentioned this pull request Sep 15, 2025

Improve handling of arrays in @defer and @stream payloads #12923

Open

jerelmiller deleted the pr/bug-reproduction-deferred-array-merge branch September 15, 2025 23:30

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	// Detect the first chunk of a deferred query and merge it with existing
	// cache data. This ensures a `cache-first` fetch policy that returns
	// partial cache data or a `cache-and-network` fetch policy that already
	// has full data in the cache does not complain when trying to merge the
	// initial deferred server data with existing cache data.
	} else if ("hasNext" in result && result.hasNext) {
	const diff = this.getDiff();
	result.data = merger.merge(diff.result, result.data);
	}

Add test that shows a bug when merging deferred results for arrays. #11374

Add test that shows a bug when merging deferred results for arrays. #11374

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phryneas commented Nov 20, 2023

Checklist:

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