Allow list-type entity @key's

docs/source/federation/advanced-features.md

@@ -54,7 +54,7 @@ type Organization {
 }
 ```
 
-> Note that although the fields argument is parsed as a selection set, some restrictions apply to make the result suitable as a key. For example, fields shouldn't return lists.
+> Note that although the fields argument is parsed as a selection set, some restrictions apply to make the result suitable as a key. For example, fields shouldn't return unions or interfaces.
 
 ## Computed fields
 

packages/apollo-federation/src/composition/validate/postComposition/__tests__/keyFieldsSelectInvalidType.test.ts

@@ -42,53 +42,6 @@ describe('keyFieldsSelectInvalidType', () => {
     expect(warnings).toHaveLength(0);
   });
 
-  it('warns if @key references fields of a list type', () => {
-    const serviceA = {
-      typeDefs: gql`
-        type Product @key(fields: "myList myOptionalList") {
-          sku: String!
-          myList: [String]!
-          myOptionalList: [String]
-          upc: String!
-          color: Color!
-        }
-
-        type Color {
-          id: ID!
-          value: String!
-        }
-      `,
-      name: 'serviceA',
-    };
-
-    const serviceB = {
-      typeDefs: gql`
-        extend type Product {
-          sku: String! @external
-          price: Int! @requires(fields: "sku")
-        }
-      `,
-      name: 'serviceB',
-    };
-
-    const { schema, errors } = composeServices([serviceA, serviceB]);
-    expect(errors).toHaveLength(0);
-
-    const warnings = validateKeyFieldsSelectInvalidType(schema);
-    expect(warnings).toMatchInlineSnapshot(`
-      Array [
-        Object {
-          "code": "KEY_FIELDS_SELECT_INVALID_TYPE",
-          "message": "[serviceA] Product -> A @key selects Product.myList, which is a list type. Keys cannot select lists.",
-        },
-        Object {
-          "code": "KEY_FIELDS_SELECT_INVALID_TYPE",
-          "message": "[serviceA] Product -> A @key selects Product.myOptionalList, which is a list type. Keys cannot select lists.",
-        },
-      ]
-    `);
-  });
-
   it('warns if @key references fields of an interface type', () => {
     const serviceA = {
       typeDefs: gql`

packages/apollo-federation/src/composition/validate/postComposition/keyFieldsSelectInvalidType.ts

@@ -2,7 +2,6 @@ import {
   GraphQLSchema,
   isObjectType,
   FieldNode,
-  isListType,
   isInterfaceType,
   isNonNullType,
   getNullableType,
@@ -46,19 +45,6 @@ export const keyFieldsSelectInvalidType = (schema: GraphQLSchema) => {
             }
 
             if (matchingField) {
-              if (
-                isListType(matchingField.type) ||
-                (isNonNullType(matchingField.type) &&
-                  isListType(getNullableType(matchingField.type)))
-              ) {
-                errors.push(
-                  errorWithCode(
-                    'KEY_FIELDS_SELECT_INVALID_TYPE',
-                    logServiceAndType(serviceName, typeName) +
-                      `A @key selects ${typeName}.${name}, which is a list type. Keys cannot select lists.`,
-                  ),
-                );
-              }
               if (
                 isInterfaceType(matchingField.type) ||
                 (isNonNullType(matchingField.type) &&

packages/apollo-gateway/src/__tests__/integration/list-key.test.ts

@@ -0,0 +1,125 @@
+import gql from 'graphql-tag';
+import { execute, ServiceDefinitionModule } from '../execution-utils';
+import { astSerializer, queryPlanSerializer } from '../../snapshotSerializers';
+
+expect.addSnapshotSerializer(astSerializer);
+expect.addSnapshotSerializer(queryPlanSerializer);
+
+const users = [
+  { id: ['1', '1'], name: 'Trevor Scheer', __typename: 'User' },
+  { id: ['2', '2'], name: 'James Baxley', __typename: 'User' },
+];
+
+const reviews = [
+  { id: '1', authorId: ['1', '1'], body: 'Good', __typename: 'Review' },
+  { id: '2', authorId: ['2', '2'], body: 'Bad', __typename: 'Review' },
+];
+
+const reviewService: ServiceDefinitionModule = {
+  name: 'review',
+  typeDefs: gql`
+    type Query {
+      reviews: [Review!]!
+    }
+
+    type Review {
+      id: ID!
+      author: User!
+      body: String!
+    }
+
+    extend type User @key(fields: "id") {
+      id: [ID!]! @external
+    }
+  `,
+  resolvers: {
+    Query: {
+      reviews() {
+        return reviews;
+      },
+    },
+    Review: {
+      author(review) {
+        return {
+          id: review.authorId,
+        };
+      },
+    },
+  },
+};
+
+const listsAreEqual = <T>(as: T[], bs: T[]) =>
+  as.length === bs.length && as.every((a, i) => bs[i] === as[i]);
+
+const userService: ServiceDefinitionModule = {
+  name: 'user',
+  typeDefs: gql`
+    type User @key(fields: "id") {
+      id: [ID!]!
+      name: String!
+    }
+  `,
+  resolvers: {
+    User: {
+      __resolveReference(reference) {
+        return users.find(user => listsAreEqual(user.id, reference.id));
+      },
+    },
+  },
+};
+
+it('fetches data correctly list type @key fields', async () => {
+  const query = gql`
+    query Reviews {
+      reviews {
+        body
+        author {
+          name
+        }
+      }
+    }
+  `;
+
+  const { data, queryPlan } = await execute([userService, reviewService], {
+    query,
+  });
+
+  expect(data).toEqual({
+    reviews: [
+      { body: 'Good', author: { name: 'Trevor Scheer' } },
+      { body: 'Bad', author: { name: 'James Baxley' } },
+    ],
+  });
+  expect(queryPlan).toMatchInlineSnapshot(`
+    QueryPlan {
+      Sequence {
+        Fetch(service: "review") {
+          {
+            reviews {
+              body
+              author {
+                __typename
+                id
+              }
+            }
+          }
+        },
+        Flatten(path: "reviews.@.author") {
+          Fetch(service: "user") {
+            {
+              ... on User {
+                __typename
+                id
+              }
+            } =>
+            {
+              ... on User {
+                name
+              }
+            }
+          },
+        },
+      },
+    }
+  `);
+});