From fda052031fd7cb18e2c70c274419b118b6803fb7 Mon Sep 17 00:00:00 2001
From: Andrew Duthie <andrew@andrewduthie.com>
Date: Wed, 25 Apr 2018 08:51:37 -0400
Subject: [PATCH] Data: Avoid tracking itemIds while request in progress

---
 core-data/queried-data/reducer.js        |  6 +++---
 core-data/queried-data/selectors.js      |  4 ++++
 core-data/queried-data/test/reducer.js   |  4 ++--
 core-data/queried-data/test/selectors.js | 26 ++++++++++++++++++++++++
 4 files changed, 35 insertions(+), 5 deletions(-)
 create mode 100644 core-data/queried-data/test/selectors.js

diff --git a/core-data/queried-data/reducer.js b/core-data/queried-data/reducer.js
index e62b177de7621..25eda4fbb4f88 100644
--- a/core-data/queried-data/reducer.js
+++ b/core-data/queried-data/reducer.js
@@ -29,7 +29,7 @@ import getQueryParts from './get-query-parts';
  *
  * @return {number[]} Merged array of item IDs.
  */
-export function getMergedItemIds( itemIds = [], nextItemIds, page, perPage ) {
+export function getMergedItemIds( itemIds, nextItemIds, page, perPage ) {
 	const nextItemIdsStartIndex = ( page - 1 ) * perPage;
 
 	// If later page has already been received, default to the larger known
@@ -125,7 +125,7 @@ const queries = flowRight( [
 	// for default query on initialization.
 	withReturnUndefinedOnUnhandledDefault,
 ] )( combineReducers( {
-	itemIds( state = [], action ) {
+	itemIds( state = null, action ) {
 		const { type, page, perPage } = action;
 
 		if ( type !== RECEIVE_ITEMS ) {
@@ -133,7 +133,7 @@ const queries = flowRight( [
 		}
 
 		return getMergedItemIds(
-			state,
+			state || [],
 			map( action.items, 'id' ),
 			page,
 			perPage
diff --git a/core-data/queried-data/selectors.js b/core-data/queried-data/selectors.js
index 3f678330cd788..a18cb2df32a51 100644
--- a/core-data/queried-data/selectors.js
+++ b/core-data/queried-data/selectors.js
@@ -46,6 +46,10 @@ export function getQueriedItems( state, query ) {
 	}
 
 	const itemIds = state.queries[ stableKey ].itemIds;
+	if ( ! itemIds ) {
+		return null;
+	}
+
 	const startOffset = ( page - 1 ) * perPage;
 	const endOffset = startOffset + perPage;
 
diff --git a/core-data/queried-data/test/reducer.js b/core-data/queried-data/test/reducer.js
index 302c363010470..6175c984e7ea2 100644
--- a/core-data/queried-data/test/reducer.js
+++ b/core-data/queried-data/test/reducer.js
@@ -16,7 +16,7 @@ import reducer, {
 
 describe( 'getMergedItemIds', () => {
 	it( 'should receive a page', () => {
-		const result = getMergedItemIds( undefined, [ 4, 5, 6 ], 2, 3 );
+		const result = getMergedItemIds( [], [ 4, 5, 6 ], 2, 3 );
 
 		expect( result ).toEqual( [
 			undefined,
@@ -164,7 +164,7 @@ describe( 'reducer', () => {
 			items: {},
 			queries: {
 				'': {
-					itemIds: [],
+					itemIds: null,
 					requestingPageByPerPage: {
 						10: {
 							1: true,
diff --git a/core-data/queried-data/test/selectors.js b/core-data/queried-data/test/selectors.js
new file mode 100644
index 0000000000000..cbad438e7ccef
--- /dev/null
+++ b/core-data/queried-data/test/selectors.js
@@ -0,0 +1,26 @@
+/**
+ * Internal dependencies
+ */
+import { getQueriedItems } from '../selectors';
+
+describe( 'getQueriedItems', () => {
+	it( 'should return null if requesting but no item IDs', () => {
+		const state = {
+			items: {},
+			queries: {
+				'': {
+					itemIds: null,
+					requestingPageByPerPage: {
+						10: {
+							1: true,
+						},
+					},
+				},
+			},
+		};
+
+		const result = getQueriedItems( state );
+
+		expect( result ).toBe( null );
+	} );
+} );