Fix alias of union types

[Jarlakxen]

I was having some issues using union types. I create a test to simulate my conditions.

      it('unions with alias', async () => {
        const mergedResult = await graphql(
          mergedSchema,
          `
            query {
              customerById(id: "c1") {
                ... on Person {
                  name
                }
                v1: vehicle {
                  ... on Bike {
                    bikeType
                  }
                }
                v2: vehicle {
                  ... on Bike {
                    bikeType
                  }
                }
              }
            }
          `,
        );

        expect(mergedResult).to.deep.equal({
          data: {
            customerById: {
              name: 'Exampler Customer',
              v1: { bikeType: 'MOUNTAIN' },
              v2: { bikeType: 'MOUNTAIN' },
            },
          },
        });
      });

This test doesn't pass but is a valid query.

I looked at the code and I found in /src/stitching/mergeSchemas.ts that the method filterSelectionSet is doing this when it find a set of fields of a type:

L612

 if (
        parentType instanceof GraphQLInterfaceType ||
        parentType instanceof GraphQLUnionType
      ) {
        selections = selections.concat({
          kind: Kind.FIELD,
          name: {
            kind: Kind.NAME,
            value: '__typename',
          },
        });
      }

So, is adding '__typename' field to the list of field requested in case of a union type or an interface. But when resolve each of field:

L584

if (
          parentType instanceof GraphQLObjectType ||
          parentType instanceof GraphQLInterfaceType
        ) {
          const fields = parentType.getFields();
          const field =
            node.name.value === '__typename'
              ? TypeNameMetaFieldDef
              : fields[node.name.value];
          if (!field) {
            return null;
          } else {
            typeStack.push(field.type);
          }
        }

Is not considering the case of a union type. I fix this situation by adding the case of the union.

I think this PR could fix #474.

TODO:

• If this PR is a new feature, 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
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[sync]

Indeed it does fix #474

[lydell]

Thanks @Jarlakxen, this saved my day!