fix(typeMerging): fix interface merging

packages/stitch/src/mergeCandidates.ts

@@ -69,11 +69,17 @@ function mergeObjectTypeCandidates(
     {}
   );
 
-  const interfaces = configs
+  const interfaceMap = configs
     .map(config => config.interfaces)
     .reduce((acc, interfaces) => {
-      return interfaces != null ? acc.concat(interfaces) : acc;
-    }, []);
+      if (interfaces != null) {
+        interfaces.forEach(iface => {
+          acc[iface.name] = iface;
+        });
+      }
+      return acc;
+    }, Object.create(null));
+  const interfaces = Object.keys(interfaceMap).map(interfaceName => interfaceMap[interfaceName]);
 
   const astNodes = pluck<ObjectTypeDefinitionNode>('astNode', candidates);
   const astNode = astNodes
@@ -157,14 +163,17 @@ function mergeInterfaceTypeCandidates(typeName: string, candidates: Array<MergeT
     {}
   );
 
-  const interfaces =
-    'interfaces' in candidates[0].type.toConfig()
-      ? configs
-          .map(config => ((config as unknown) as { interfaces: Array<GraphQLInterfaceType> }).interfaces)
-          .reduce((acc, interfaces) => {
-            return interfaces != null ? acc.concat(interfaces) : acc;
-          }, [])
-      : undefined;
+  const interfaceMap = configs
+    .map(config => ((config as unknown) as { interfaces: Array<GraphQLInterfaceType> }).interfaces)
+    .reduce((acc, interfaces) => {
+      if (interfaces != null) {
+        interfaces.forEach(iface => {
+          acc[iface.name] = iface;
+        });
+      }
+      return acc;
+    }, Object.create(null));
+  const interfaces = Object.keys(interfaceMap).map(interfaceName => interfaceMap[interfaceName]);
 
   const astNodes = pluck<InterfaceTypeDefinitionNode>('astNode', candidates);
   const astNode = astNodes
@@ -196,7 +205,13 @@ function mergeUnionTypeCandidates(typeName: string, candidates: Array<MergeTypeC
   const description = descriptions[descriptions.length - 1];
 
   const configs = candidates.map(candidate => (candidate.type as GraphQLUnionType).toConfig());
-  const types = configs.reduce((acc, config) => acc.concat(config.types), []);
+  const typeMap = configs.reduce((acc, config) => {
+    config.types.forEach(type => {
+      typeMap[type.name] = type;
+    });
+    return acc;
+  }, Object.create(null));
+  const types = Object.keys(typeMap).map(typeName => typeMap[typeName]);
 
   const astNodes = pluck<UnionTypeDefinitionNode>('astNode', candidates);
   const astNode = astNodes

packages/stitch/tests/mergeInterfaces.test.ts

@@ -0,0 +1,82 @@
+import { graphql } from 'graphql';
+import { makeExecutableSchema } from '@graphql-tools/schema';
+import { stitchSchemas } from '../src/stitchSchemas';
+
+describe('merging interfaces', () => {
+  test('merges interfaces across subschemas', async () => {
+    const namedItemSchema = makeExecutableSchema({
+      typeDefs: `
+        interface Placement {
+          id: ID!
+          name: String!
+        }
+        type Item implements Placement {
+          id: ID!
+          name: String!
+        }
+        type Query {
+          itemById(id: ID!): Item
+        }
+      `,
+      resolvers: {
+        Query: {
+          itemById(_obj, args) {
+            return { id: args.id, name: `Item ${args.id}` };
+          }
+        }
+      }
+    });
+
+    const indexedItemSchema = makeExecutableSchema({
+      typeDefs: `
+        interface Placement {
+          id: ID!
+          index: Int!
+        }
+        type Item implements Placement {
+          id: ID!
+          index: Int!
+        }
+        type Query {
+          placementById(id: ID!): Placement
+        }
+      `,
+      resolvers: {
+        Query: {
+          placementById(_obj, args) {
+            return { __typename: 'Item', id: args.id, index: Number(args.id) };
+          }
+        }
+      }
+    });
+
+    const stitchedSchema = stitchSchemas({
+      subschemas: [
+        {
+          schema: namedItemSchema,
+          merge: {
+            Item: {
+              selectionSet: '{ id }',
+              fieldName: 'itemById',
+              args: ({ id }) => ({ id }),
+            }
+          }
+        },
+        { schema: indexedItemSchema },
+      ],
+      mergeTypes: true,
+    });
+
+    const result = await graphql(stitchedSchema, `
+      query {
+        placement: placementById(id: 23) {
+          id
+          index
+          name
+        }
+      }
+    `);
+
+    expect(result.data.placement).toEqual({ id: '23', index: 23, name: 'Item 23' });
+  });
+});