Require opting into SchemaLink validation.

Follow-up to #7094, to avoid enabling validation by default, since
@apollo/client@3.3.0 is not a major release.

CHANGELOG.md

@@ -75,7 +75,7 @@
 - In addition to the `result.data` property, `useQuery` and `useLazyQuery` will now provide a `result.previousData` property, which can be useful when a network request is pending and `result.data` is undefined, since `result.previousData` can be rendered instead of rendering an empty/loading state. <br/>
   [@hwillson](https://github.com/hwillson) in [#7082](https://github.com/apollographql/apollo-client/pull/7082)
 
-- The schema link package (`@apollo/client/link/schema`) will now validate incoming queries against its client-side schema, and return `errors` as a GraphQL server would. <br/>
+- Passing `validate: true` to the `SchemaLink` constructor will enable validation of incoming queries against the local schema before execution, returning validation errors in `result.errors`, just like a non-local GraphQL endpoint typically would. <br/>
   [@amannn](https://github.com/amannn) in [#7094](https://github.com/apollographql/apollo-client/pull/7094)
 
 - Allow optional arguments in `keyArgs: [...]` arrays for `InMemoryCache` field policies. <br/>

src/link/schema/__tests__/schemaLink.ts

@@ -65,17 +65,19 @@ describe('SchemaLink', () => {
   });
 
   it('calls error when fetch fails', done => {
-    const schema = makeExecutableSchema({
-      typeDefs,
-      resolvers: {
-        Query: {
-          sampleQuery() {
-            throw new Error('Unauthorized');
+    const link = new SchemaLink({
+      validate: true,
+      schema: makeExecutableSchema({
+        typeDefs,
+        resolvers: {
+          Query: {
+            sampleQuery() {
+              throw new Error('Unauthorized');
+            }
           }
         }
-      }
+      }),
     });
-    const link = new SchemaLink({ schema });
     const observable = execute(link, {
       query: sampleQuery,
     });
@@ -199,8 +201,12 @@ describe('SchemaLink', () => {
   });
 
   it('reports errors for unknown queries', done => {
-    const schema = makeExecutableSchema({typeDefs})
-    const link = new SchemaLink({ schema });
+    const link = new SchemaLink({
+      validate: true,
+      schema: makeExecutableSchema({
+        typeDefs,
+      }),
+    });
     const observable = execute(link, {
       query: gql`
         query {

src/link/schema/index.ts

@@ -24,19 +24,27 @@ export namespace SchemaLink {
      * A context to provide to resolvers declared within the schema.
      */
     context?: ResolverContext | ResolverContextFunction;
+
+    /**
+     * Validate incoming queries against the given schema, returning
+     * validation errors as a GraphQL server would.
+     */
+    validate?: boolean;
   }
 }
 
 export class SchemaLink extends ApolloLink {
   public schema: SchemaLink.Options["schema"];
   public rootValue: SchemaLink.Options["rootValue"];
   public context: SchemaLink.Options["context"];
+  public validate: boolean;
 
   constructor(options: SchemaLink.Options) {
     super();
     this.schema = options.schema;
     this.rootValue = options.rootValue;
     this.context = options.context;
+    this.validate = !!options.validate;
   }
 
   public request(operation: Operation): Observable<FetchResult> {
@@ -48,9 +56,11 @@ export class SchemaLink extends ApolloLink {
             : this.context
         )
       ).then(context => {
-        const validationErrors = validate(this.schema, operation.query);
-        if (validationErrors.length > 0) {
-          return { errors: validationErrors };
+        if (this.validate) {
+          const validationErrors = validate(this.schema, operation.query);
+          if (validationErrors.length > 0) {
+            return { errors: validationErrors };
+          }
         }
 
         return execute(