fix(gateway): merge selection sets of @requires fields (apollograph…

…ql/apollo-server#4064)

Previously, it was possible for the gateway to build and issue queries
for resolving fields that resembled the following:

```graphql
{
  product { id }
  product { price }
}
```

In the above example, the second selection set will take precedence and
the first ignored, meaning the query _ought_ to be:

```graphql
{
  product { id price }
}
```

This is accomplished in this PR by merging the selection sets recursively.

fixes apollographql/apollo-server#3848

Co-authored-by: Lenny Burdette <lennyburdette@gmail.com>
Apollo-Orig-Commit-AS: apollographql/apollo-server@3cdcd06

gateway-js/CHANGELOG.md

@@ -4,7 +4,8 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- __FIX__: Collapse nested required fields into a single body in the query plan. Before, some nested fields' selection sets were getting split, causing some of their subfields to be dropped when executing the query. This fix collapses the split selection sets into one. [#4064](https://github.com/apollographql/apollo-server/pull/4064)
+
 
 ## 0.16.0
 

gateway-js/src/FieldSet.ts

@@ -10,6 +10,7 @@ import {
   GraphQLObjectType,
 } from 'graphql';
 import { getResponseName } from './utilities/graphql';
+import { partition, groupBy } from './utilities/array';
 
 export interface Field<
   TParent extends GraphQLCompositeType = GraphQLCompositeType
@@ -45,25 +46,6 @@ export function matchesField(field: Field) {
   };
 }
 
-function groupBy<T, U>(keyFunction: (element: T) => U) {
-  return (iterable: Iterable<T>) => {
-    const result = new Map<U, T[]>();
-
-    for (const element of iterable) {
-      const key = keyFunction(element);
-      const group = result.get(key);
-
-      if (group) {
-        group.push(element);
-      } else {
-        result.set(key, [element]);
-      }
-    }
-
-    return result;
-  };
-}
-
 export const groupByResponseName = groupBy<Field, string>(field =>
   getResponseName(field.fieldNode)
 );
@@ -147,6 +129,38 @@ function mergeSelectionSets(fieldNodes: FieldNode[]): SelectionSetNode {
 
   return {
     kind: 'SelectionSet',
-    selections,
+    selections: mergeFieldNodeSelectionSets(selections),
   };
 }
+
+function mergeFieldNodeSelectionSets(
+  selectionNodes: SelectionNode[],
+): SelectionNode[] {
+  const [fieldNodes, fragmentNodes] = partition(
+    selectionNodes,
+    (node): node is FieldNode => node.kind === Kind.FIELD,
+  );
+
+  const [aliasedFieldNodes, nonAliasedFieldNodes] = partition(
+    fieldNodes,
+    node => !!node.alias,
+  );
+
+  const mergedFieldNodes = Array.from(
+    groupBy((node: FieldNode) => node.name.value)(
+      nonAliasedFieldNodes,
+    ).values(),
+  ).map((nodesWithSameName) => {
+    const node = nodesWithSameName[0];
+    if (node.selectionSet) {
+      node.selectionSet.selections = mergeFieldNodeSelectionSets(
+        nodesWithSameName.flatMap(
+          (node) => node.selectionSet?.selections || [],
+        ),
+      );
+    }
+    return node;
+  });
+
+  return [...mergedFieldNodes, ...aliasedFieldNodes, ...fragmentNodes];
+}

gateway-js/src/__tests__/integration/complex-key.test.ts

@@ -171,7 +171,6 @@ it('works fetches data correctly with complex / nested @key fields', async () =>
                 organization {
                   id
                   __typename
-                  id
                 }
               }
             }

gateway-js/src/__tests__/integration/requires.test.ts

@@ -1,4 +1,6 @@
+import gql from 'graphql-tag';
 import { execute } from '../execution-utils';
+import { serializeQueryPlan } from '../..';
 
 it('supports passing additional fields defined by a requires', async () => {
   const query = `#graphql
@@ -38,3 +40,228 @@ it('supports passing additional fields defined by a requires', async () => {
   expect(queryPlan).toCallService('product');
   expect(queryPlan).toCallService('books');
 });
+
+const serviceA = {
+  name: 'a',
+  typeDefs: gql`
+    type Query {
+      user: User
+    }
+
+    type User @key(fields: "id") {
+      id: ID!
+      preferences: Preferences
+    }
+
+    type Preferences {
+      favorites: Things
+    }
+
+    type Things {
+      color: String
+      animal: String
+    }
+  `,
+  resolvers: {
+    Query: {
+      user() {
+        return {
+          id: '1',
+          preferences: {
+            favorites: { color: 'limegreen', animal: 'platypus' },
+          },
+        };
+      },
+    },
+  },
+};
+
+const serviceB = {
+  name: 'b',
+  typeDefs: gql`
+    extend type User @key(fields: "id") {
+      id: ID! @external
+      preferences: Preferences @external
+      favoriteColor: String
+        @requires(fields: "preferences { favorites { color } }")
+      favoriteAnimal: String
+        @requires(fields: "preferences { favorites { animal } }")
+    }
+
+    extend type Preferences {
+      favorites: Things @external
+    }
+
+    extend type Things {
+      color: String @external
+      animal: String @external
+    }
+  `,
+  resolvers: {
+    User: {
+      favoriteColor(user: any) {
+        return user.preferences.favorites.color;
+      },
+      favoriteAnimal(user: any) {
+        return user.preferences.favorites.animal;
+      },
+    },
+  },
+};
+
+it('collapses nested requires', async () => {
+  const query = `#graphql
+    query UserFavorites {
+      user {
+        favoriteColor
+        favoriteAnimal
+      }
+    }
+  `;
+
+  const { data, errors, queryPlan } = await execute(
+    {
+      query,
+    },
+    [serviceA, serviceB],
+  );
+
+  expect(errors).toEqual(undefined);
+
+  expect(serializeQueryPlan(queryPlan)).toMatchInlineSnapshot(`
+    "QueryPlan {
+      Sequence {
+        Fetch(service: \\"a\\") {
+          {
+            user {
+              __typename
+              id
+              preferences {
+                favorites {
+                  color
+                  animal
+                }
+              }
+            }
+          }
+        },
+        Flatten(path: \\"user\\") {
+          Fetch(service: \\"b\\") {
+            {
+              ... on User {
+                __typename
+                id
+                preferences {
+                  favorites {
+                    color
+                    animal
+                  }
+                }
+              }
+            } =>
+            {
+              ... on User {
+                favoriteColor
+                favoriteAnimal
+              }
+            }
+          },
+        },
+      },
+    }"
+  `);
+
+  expect(data).toEqual({
+    user: {
+      favoriteAnimal: 'platypus',
+      favoriteColor: 'limegreen',
+    },
+  });
+
+  expect(queryPlan).toCallService('a');
+  expect(queryPlan).toCallService('b');
+});
+
+it('collapses nested requires with user-defined fragments', async () => {
+  const query = `#graphql
+    query UserFavorites {
+      user {
+        favoriteAnimal
+        ...favoriteColor
+      }
+    }
+
+    fragment favoriteColor on User {
+      preferences {
+        favorites {
+          color
+        }
+      }
+    }
+  `;
+
+  const { data, errors, queryPlan } = await execute(
+    {
+      query,
+    },
+    [serviceA, serviceB],
+  );
+
+  expect(errors).toEqual(undefined);
+
+  expect(serializeQueryPlan(queryPlan)).toMatchInlineSnapshot(`
+    "QueryPlan {
+      Sequence {
+        Fetch(service: \\"a\\") {
+          {
+            user {
+              __typename
+              id
+              preferences {
+                favorites {
+                  animal
+                  color
+                }
+              }
+            }
+          }
+        },
+        Flatten(path: \\"user\\") {
+          Fetch(service: \\"b\\") {
+            {
+              ... on User {
+                __typename
+                id
+                preferences {
+                  favorites {
+                    animal
+                    color
+                  }
+                }
+              }
+            } =>
+            {
+              ... on User {
+                favoriteAnimal
+              }
+            }
+          },
+        },
+      },
+    }"
+  `);
+
+  expect(data).toEqual({
+    user: {
+      favoriteAnimal: 'platypus',
+      preferences: {
+        favorites: {
+          color: 'limegreen',
+        },
+      },
+    },
+  });
+
+  expect(queryPlan).toCallService('a');
+  expect(queryPlan).toCallService('b');
+});

gateway-js/src/utilities/array.ts

@@ -16,6 +16,14 @@ export function compactMap<T, U>(
   );
 }
 
+export function partition<T, U extends T>(
+  array: T[],
+  predicate: (element: T, index: number, array: T[]) => element is U,
+): [U[], T[]];
+export function partition<T>(
+  array: T[],
+  predicate: (element: T, index: number, array: T[]) => boolean,
+): [T[], T[]];
 export function partition<T>(
   array: T[],
   predicate: (element: T, index: number, array: T[]) => boolean,
@@ -48,3 +56,22 @@ export function findAndExtract<T>(
 
   return [array[index], remaining];
 }
+
+export function groupBy<T, U>(keyFunction: (element: T) => U) {
+  return (iterable: Iterable<T>) => {
+    const result = new Map<U, T[]>();
+
+    for (const element of iterable) {
+      const key = keyFunction(element);
+      const group = result.get(key);
+
+      if (group) {
+        group.push(element);
+      } else {
+        result.set(key, [element]);
+      }
+    }
+
+    return result;
+  };
+}