Nested Fragment on Union type field removes data from response

When using nested fragments on a union type, the properties from the subfragments are removed from the response without warning or error, even though it is returned from the server.

Intended outcome: Nested fragments on union types should return the data as requested in the GraphQL query.

Actual outcome: Currently, the data objects in the response are replaced with an object that removes all properties but the __typename:

{"__typename": "Subtype"}

How to reproduce the issue:

Run a query against any GraphQL API that returns a union type with something like the following:

Partial Schema:

union UnionType = Subtype

type Subtype {
  id
  text
}

type Query {
  getUnion: UnionType
}

import {ApolloClient, gql, HttpLink, InMemoryCache} from '@apollo/client';
import fetch from "cross-fetch";

(async () => {
	const client = new ApolloClient({
		ssrMode: typeof window === 'undefined',
		link: new HttpLink({
			fetch,
			uri: 'https://graphqlapi-with-union-type.com/graphql', // Server URL (must be absolute)
			credentials: 'same-origin', // Additional fetch() options like `credentials` or `headers`
		}),
		cache: new InMemoryCache(),
	});

  const result = await client.query({
    query: gql`
      query Q {
        getUnion {
            __typename
            ...UnionFragment
            # If we add the SubtypeFragment here too, it returns the correct data, but not without that fragment:
            # ...SubtypeFragment
        }
      }

      fragment UnionFragment on UnionType {
          __typename
          ...SubtypeFragment
      }

      fragment SubtypeFragment on Subtype {
        __typename
        id
        text
      }
		`,
  });
	console.log(JSON.stringify(result.data, null, 2));
})();

When the fragment of the subtype is added to the root field, the correct data is returned, but this should work without explicitly adding all subfragments IMO.

Versions Binaries: Node: 10.22.0 - ~/.nvm/versions/node/v10.22.0/bin/node npm: 6.14.6 - ~/.nvm/versions/node/v10.22.0/bin/npm Browsers: Chrome: 85.0.4183.102 Firefox: 80.0.1 Safari: 13.1.2 npmPackages: @apollo/client: ^3.2.0 => 3.2.0 apollo: ^2.27.0 => 2.27.0

[benjamn]

@ivome Have you tried providing possibleTypes to the InMemoryCache constructor? You don’t have to specify every possible subtype relationship in your schema, but you do need to specify the ones that matter for fragment matching.

[ivome]

@benjamn That does the trick, thank you!

Would be great to get some kind of error message though when apollo client removes data from the response in such a case.