Allow cache.identify to take Reference objects. (#6447)

There are a number of scenarios where an object found in the cache will
have the union type `StoreObject | Reference`. Rather than forcing
application code to check isReference and access the reference.__ref
property manually, we can simply make cache.identify work for both
StoreObject and Reference objects.

src/cache/core/cache.ts

@@ -66,7 +66,7 @@ export abstract class ApolloCache<TSerialized> implements DataProxy {
     return document;
   }
 
-  public identify(object: StoreObject): string | undefined {
+  public identify(object: StoreObject | Reference): string | undefined {
     return;
   }
 

src/cache/inmemory/__tests__/entityStore.ts

@@ -4,7 +4,7 @@ import { InMemoryCache } from '../inMemoryCache';
 import { DocumentNode } from 'graphql';
 import { StoreObject } from '../types';
 import { ApolloCache } from '../../core/cache';
-import { Reference } from '../../../utilities/graphql/storeUtils';
+import { Reference, makeReference, isReference } from '../../../utilities/graphql/storeUtils';
 import { MissingFieldError } from '../..';
 
 describe('EntityStore', () => {
@@ -1417,6 +1417,39 @@ describe('EntityStore', () => {
     });
   });
 
+  it("supports cache.identify(reference)", () => {
+    const cache = new InMemoryCache({
+      typePolicies: {
+        Task: {
+          keyFields: ["uuid"],
+        },
+      },
+    });
+
+    expect(cache.identify(makeReference("oyez"))).toBe("oyez");
+
+    const todoRef = cache.writeFragment({
+      fragment: gql`fragment TodoId on Todo { id }`,
+      data: {
+        __typename: "Todo",
+        id: 123,
+      },
+    });
+    expect(isReference(todoRef)).toBe(true);
+    expect(cache.identify(todoRef!)).toBe("Todo:123");
+
+    const taskRef = cache.writeFragment({
+      fragment: gql`fragment TaskId on Task { id }`,
+      data: {
+        __typename: "Task",
+        uuid: "eb8cffcc-7a9e-4d8b-a517-7d987bf42138",
+      },
+    });
+    expect(isReference(taskRef)).toBe(true);
+    expect(cache.identify(taskRef!)).toBe(
+      'Task:{"uuid":"eb8cffcc-7a9e-4d8b-a517-7d987bf42138"}');
+  });
+
   it("supports cache.identify(object)", () => {
     const queryWithAliases: DocumentNode = gql`
       query {

src/cache/inmemory/inMemoryCache.ts

@@ -7,7 +7,7 @@ import { dep, wrap } from 'optimism';
 import { ApolloCache, Transaction } from '../core/cache';
 import { Cache } from '../core/types/Cache';
 import { addTypenameToDocument } from '../../utilities/graphql/transform';
-import { StoreObject, Reference }  from '../../utilities/graphql/storeUtils';
+import { StoreObject, Reference, isReference }  from '../../utilities/graphql/storeUtils';
 import {
   ApolloReducerConfig,
   NormalizedCacheObject,
@@ -206,8 +206,10 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   // the object must contain a __typename and any primary key fields required
   // to identify entities of that type. If you pass a query result object, be
   // sure that none of the primary key fields have been renamed by aliasing.
-  public identify(object: StoreObject): string | undefined {
-    return this.policies.identify(object)[0];
+  // If you pass a Reference object, its __ref ID string will be returned.
+  public identify(object: StoreObject | Reference): string | undefined {
+    return isReference(object) ? object.__ref :
+      this.policies.identify(object)[0];
   }
 
   public evict(options: Cache.EvictOptions): boolean {