fix(stitch) fix abstract type merge failure (#1981)

fix in #1917 includes regression

packages/stitch/src/mergeCandidates.ts

@@ -207,7 +207,7 @@ function mergeUnionTypeCandidates(typeName: string, candidates: Array<MergeTypeC
   const configs = candidates.map(candidate => (candidate.type as GraphQLUnionType).toConfig());
   const typeMap = configs.reduce((acc, config) => {
     config.types.forEach(type => {
-      typeMap[type.name] = type;
+      acc[type.name] = type;
     });
     return acc;
   }, Object.create(null));

packages/stitch/tests/mergeAbstractTypes.test.ts

@@ -0,0 +1,95 @@
+import { makeExecutableSchema } from '@graphql-tools/schema';
+import { stitchSchemas } from '@graphql-tools/stitch';
+import { graphql } from 'graphql';
+
+describe('Abstract type merge', () => {
+  it('merges with abstract type definitions', async () => {
+    const imageSchema = makeExecutableSchema({
+      typeDefs: `
+        type Image {
+          id: ID!
+          url: String!
+        }
+        type Query {
+          image(id: ID!): Image
+        }
+      `,
+      resolvers: {
+        Query: {
+          image: (root, { id }) => ({ id, url: `/path/to/${id}` }),
+        }
+      }
+    });
+
+    const contentSchema = makeExecutableSchema({
+      typeDefs: `
+        type Post {
+          id: ID!
+          leadArt: LeadArt
+        }
+        type Image {
+          id: ID!
+        }
+        type Video {
+          id: ID!
+          url: String!
+        }
+        union LeadArt = Image | Video
+        type Query {
+          post(id: ID!): Post
+        }
+      `,
+      resolvers: {
+        Query: {
+          post: (root, { id }) => ({ id, leadArt: { __typename: 'Image', id: '23' } }),
+        }
+      }
+    });
+
+    const gatewaySchema = stitchSchemas({
+      subschemas: [
+        {
+          schema: imageSchema,
+          merge: {
+            Image: {
+              selectionSet: '{ id }',
+              fieldName: 'image',
+              args: ({ id }) => ({ id }),
+            },
+          },
+        },
+        {
+          schema: contentSchema,
+          merge: {
+            Post: {
+              selectionSet: '{ id }',
+              fieldName: 'post',
+              args: ({ id }) => ({ id }),
+            },
+          },
+        },
+      ],
+      mergeTypes: true
+    });
+
+    const { data } = await graphql(gatewaySchema, `
+      query {
+        post(id: 55) {
+          leadArt {
+            __typename
+            ...on Image {
+              id
+              url
+            }
+          }
+        }
+      }
+    `);
+
+    expect(data.post.leadArt).toEqual({
+      __typename: 'Image',
+      url: '/path/to/23',
+      id: '23',
+    });
+  });
+});