Using @graphql-tools/mock with buildSubgraphSchema breaks _Entity

[michael-watson]

Bug Description

buildSubgraphSchema does not provide a __resolveType resolver for _Entity. @graphql-tools/mock provides a function addMocksToSchema that copies an existing schema into a new GraphQLSchema with mocked resolvers. There is a preserveResolvers config option that is meant to keep any existing resolvers, but the preserved elements may still be copied into a new schema object; specifically, unions that don't have resolveType defined.

Since buildSubgraphSchema does not provide a __resolveType for Entity, a new GraphQLUnionType is used based on the toConfig() results and the resolveType - this is just standard graphql-js and the _Entity union should copy over. You can view the relevant code in @graphql-tools/mock here.

Related Issues and PRs
I believe this is also the root of #411 and #348. Another PR addresses the other issue of __resolveReference resolvers being dropped in a similar process with toConfig; that PR is needed for mocks to work and call the appropriate entities __resolveRefernce

Reproduction

Codesandbox example, example query:

query($representations: [_Any!]!) {
  _entities(representations: $representations) {
    ... on Foo {
      name
    }
  }
}

# Variables
{
  representations: [{ __typename: "Foo", id: "1" }]
}

Will give the following error (you can run the above query in sandbox:

{
  errors: [
    [GraphQLError: Abstract type "_Entity" must resolve to an Object type at runtime forfield "Query._entities". Either the "_Entity" type should provide a "resolveType" function or each possible type should provide an "isTypeOf" function.] {
      locations: [Array],
      path: [Array],
      extensions: [Object]
    }
  ]
}

Workaround

You can provide the _Entity.__resolveType resolver to addMocksToSchema as shown below and in this codesandbox reproduction:

const schema = addMocksToSchema({
  schema: buildSubgraphSchema({ typeDefs }),
  preserveResolvers: true,
  mocks: {},
  resolvers: {
    _Entity: {
      __resolveType(parent) {
        return parent.__typename;
      },
    },
  },
});

The error will be gone, unfortunately you won't get any data back because of the issue around __resolveRefernce, but this is fixed in this PR and should release in the future 🎉

(I couldn't figure out how to install the tarbill in code sandbox, but it worked locally in my identical sample to codesandbox)

[trevor-scheer]

Fixed via #1773