fix(typeMerging): field selection sets (#1958)

should work on their own when necessary

see: #1956

packages/delegate/src/results/mergeFields.ts

@@ -26,23 +26,27 @@ const sortSubschemasByProxiability = memoize4(function (
   targetSubschemas.forEach(t => {
     const selectionSet = mergedTypeInfo.selectionSets.get(t);
     const fieldSelectionSets = mergedTypeInfo.fieldSelectionSets.get(t);
-    if (!subschemaTypesContainSelectionSet(mergedTypeInfo, sourceSubschemaOrSourceSubschemas, selectionSet)) {
-      nonProxiableSubschemas.push(t);
-    } else if (fieldSelectionSets == null) {
-      proxiableSubschemas.push(t);
-    } else if (
-      fieldNodes.every(fieldNode => {
-        const fieldName = fieldNode.name.value;
-        const fieldSelectionSet = fieldSelectionSets[fieldName];
-        return (
-          fieldSelectionSet == null ||
-          subschemaTypesContainSelectionSet(mergedTypeInfo, sourceSubschemaOrSourceSubschemas, fieldSelectionSet)
-        );
-      })
+    if (
+      selectionSet != null &&
+      !subschemaTypesContainSelectionSet(mergedTypeInfo, sourceSubschemaOrSourceSubschemas, selectionSet)
     ) {
-      proxiableSubschemas.push(t);
-    } else {
       nonProxiableSubschemas.push(t);
+    } else {
+      if (
+        fieldSelectionSets == null ||
+        fieldNodes.every(fieldNode => {
+          const fieldName = fieldNode.name.value;
+          const fieldSelectionSet = fieldSelectionSets[fieldName];
+          return (
+            fieldSelectionSet == null ||
+            subschemaTypesContainSelectionSet(mergedTypeInfo, sourceSubschemaOrSourceSubschemas, fieldSelectionSet)
+          );
+        })
+      ) {
+        proxiableSubschemas.push(t);
+      } else {
+        nonProxiableSubschemas.push(t);
+      }
     }
   });
 

packages/stitch/tests/fieldSelectionSets.test.ts

@@ -0,0 +1,117 @@
+import { graphql } from 'graphql';
+import { makeExecutableSchema } from '@graphql-tools/schema';
+import { stitchSchemas } from '@graphql-tools/stitch';
+
+describe('type merging with only field selection sets', () => {
+  const listingsSchema = makeExecutableSchema({
+    typeDefs: `
+      type Listing {
+        id: ID!
+        sellerId: ID!
+        buyerId: ID
+      }
+      type Query {
+        listing(id: ID!): Listing
+      }
+    `,
+    resolvers: {
+      Query: {
+        listing: () => ({ id: 1, buyerId: 2, sellerId: 3 })
+      },
+    },
+  });
+
+  const usersSchema = makeExecutableSchema({
+    typeDefs: `
+      type User {
+        id: ID!
+        userName: String
+      }
+      type Listing {
+        seller: User!
+        buyer: User
+      }
+      input ListingKeys {
+        sellerId: ID
+        buyerId: ID
+      }
+      type Query {
+        _listings(keys: [ListingKeys!]!): [Listing]!
+      }
+    `,
+    resolvers: {
+      Query: {
+        _listings: (_root, args) => args.keys,
+      },
+      Listing: {
+        buyer: (_root, args) => ({ id: args.buyerId, userName: 'Bob' }),
+        seller: (_root, args) => ({ id: args.sellerId, userName: 'Tom' }),
+      }
+    },
+  });
+
+  const stitchedSchema = stitchSchemas({
+    subschemas: [
+      {
+        schema: listingsSchema,
+        merge: {
+          Listing: {
+            selectionSet: '{ id }',
+            fieldName: 'listing',
+            args: ({ id }) => ({ id })
+          }
+        }
+      },
+      {
+        schema: usersSchema,
+        merge: {
+          Listing: {
+            // selectionSet: '{ id }', // <~ required? We don't need this data for anything here
+            fields: {
+              buyer: { selectionSet: '{ buyerId }' },
+              seller: { selectionSet: '{ sellerId }' },
+            },
+            fieldName: '_listings',
+            key: ({ buyerId, sellerId }) => ({ buyerId, sellerId }),
+            argsFromKeys: (keys) => ({ keys }),
+          }
+        }
+      }
+    ],
+    mergeTypes: true,
+  });
+
+  test('errors without selectionSet...', async () => {
+    const result = await graphql(
+      stitchedSchema, `
+        query {
+          listing(id: 23) {
+            id
+            buyer {
+              userName
+            }
+            seller {
+              userName
+            }
+          }
+        }
+      `,
+    );
+
+    const expectedResult = {
+      data: {
+        listing: {
+          id: '1',
+          buyer: {
+            userName: 'Bob',
+          },
+          seller: {
+            userName: 'Tom',
+          }
+        },
+      },
+    };
+
+    expect(result).toEqual(expectedResult);
+  });
+});