fix errors

packages/delegate/src/externalObjects.ts

@@ -42,7 +42,7 @@ export function mergeExternalObjects(
   let errors: Array<GraphQLError> = [];
 
   sources.forEach((source, index) => {
-    if (source instanceof GraphQLError) {
+    if (source instanceof GraphQLError || source === null) {
       const selectionSet = selectionSets[index];
       const fieldNodes = collectFields(
         {
@@ -57,7 +57,8 @@ export function mergeExternalObjects(
       );
       const nullResult = {};
       Object.keys(fieldNodes).forEach(responseKey => {
-        nullResult[responseKey] = relocatedError(source, path.concat([responseKey]));
+        nullResult[responseKey] =
+          source instanceof GraphQLError ? relocatedError(source, path.concat([responseKey])) : null;
       });
       results.push(nullResult);
     } else {

packages/stitch/tests/mergeFailures.test.ts

@@ -0,0 +1,163 @@
+import { makeExecutableSchema } from '@graphql-tools/schema';
+import { stitchSchemas } from '@graphql-tools/stitch';
+import { ExecutionResult } from '@graphql-tools/utils';
+import { graphql, GraphQLError, GraphQLSchema } from 'graphql';
+
+describe('merge failures', () => {
+  const firstSchema = makeExecutableSchema({
+    typeDefs: `
+      type Thing {
+        id: ID!
+        name: String!
+      }
+      type Query {
+        thing(id: ID!): Thing
+      }
+    `,
+    resolvers: {
+      Query: {
+        thing: (_root, { id }) => ({ id, name: 'The Thing' }),
+      }
+    }
+  });
+
+  const getGatewaySchema = (secondSchema: GraphQLSchema): GraphQLSchema => stitchSchemas({
+    subschemas: [
+      {
+        schema: firstSchema,
+        merge: {
+          Thing: {
+            selectionSet: '{ id }',
+            fieldName: 'thing',
+            args: ({ id }) => ({ id }),
+          }
+        }
+      },
+      {
+        schema: secondSchema,
+        merge: {
+          Thing: {
+            selectionSet: '{ id }',
+            fieldName: '_thing',
+            args: ({ id }) => ({ id }),
+          }
+        }
+      },
+    ],
+    mergeTypes: true
+  });
+
+  it('proxies merged errors', async () => {
+    const secondSchema = makeExecutableSchema({
+      typeDefs: `
+        type Thing {
+          id: ID!
+          description: String!
+        }
+        type Query {
+          _thing(id: ID!): Thing
+        }
+      `,
+      resolvers: {
+        Query: {
+          _thing: () => new Error('unable to produce the thing'),
+        }
+      }
+    });
+
+    const gatewaySchema = getGatewaySchema(secondSchema);
+
+    const result = await graphql(gatewaySchema, `
+      query {
+        thing(id: 23) {
+          id
+          name
+          description
+        }
+      }
+    `);
+
+    const expectedResult: ExecutionResult = {
+      data: { thing: null },
+      errors: [new GraphQLError('unable to produce the thing')],
+    }
+
+    expect(result).toEqual(expectedResult);
+  });
+
+  it('proxies inappropriate null', async () => {
+    const secondSchema = makeExecutableSchema({
+      typeDefs: `
+        type Thing {
+          id: ID!
+          description: String!
+        }
+        type Query {
+          _thing(id: ID!): Thing
+        }
+      `,
+      resolvers: {
+        Query: {
+          _thing: () => null,
+        }
+      }
+    });
+
+    const gatewaySchema = getGatewaySchema(secondSchema);
+
+    const result = await graphql(gatewaySchema, `
+      query {
+        thing(id: 23) {
+          id
+          name
+          description
+        }
+      }
+    `);
+
+    const expectedResult: ExecutionResult = {
+      data: { thing: null },
+      errors: [new GraphQLError('Cannot return null for non-nullable field Thing.description.')],
+    }
+
+    expect(result).toEqual(expectedResult);
+  });
+
+  it('proxies errors on object', async () => {
+    const secondSchema = makeExecutableSchema({
+      typeDefs: `
+        type Thing {
+          id: ID!
+          description: String!
+        }
+        type Query {
+          _thing(id: ID!): Thing
+        }
+      `,
+      resolvers: {
+        Query: {
+          _thing: () => ({}),
+        }
+      }
+    });
+
+    const gatewaySchema = getGatewaySchema(secondSchema);
+
+    const result = await graphql(gatewaySchema, `
+      query {
+        thing(id: 23) {
+          id
+          name
+          description
+        }
+      }
+    `);
+
+    const expectedResult: ExecutionResult = {
+      data: { thing: null },
+      errors: [new GraphQLError('Cannot return null for non-nullable field Thing.description.')],
+    }
+
+    expect(result).toEqual(expectedResult);
+  });
+});