From 25fed77a6ad0db185a732af1c0f0f417ca024805 Mon Sep 17 00:00:00 2001
From: Ben Newman <ben@apollographql.com>
Date: Mon, 28 Jun 2021 19:15:26 -0400
Subject: [PATCH] Prefer existing.pageInfo cursors in relayStylePagination read
 function.

---
 src/cache/inmemory/__tests__/policies.ts      |   2 +-
 .../__tests__/relayStylePagination.test.ts    | 109 +++++++++++++++++-
 src/utilities/policies/pagination.ts          |  17 ++-
 3 files changed, 122 insertions(+), 6 deletions(-)

diff --git a/src/cache/inmemory/__tests__/policies.ts b/src/cache/inmemory/__tests__/policies.ts
index 58fc8032af1..3fe2c070e51 100644
--- a/src/cache/inmemory/__tests__/policies.ts
+++ b/src/cache/inmemory/__tests__/policies.ts
@@ -3462,7 +3462,7 @@ describe("type policies", function () {
                 edges,
                 pageInfo: {
                   __typename: "PageInfo",
-                  startCursor: thirdPageInfo.startCursor,
+                  startCursor: fourthPageInfo.startCursor,
                   endCursor: fifthPageInfo.endCursor,
                   hasPreviousPage: false,
                   hasNextPage: true,
diff --git a/src/utilities/policies/__tests__/relayStylePagination.test.ts b/src/utilities/policies/__tests__/relayStylePagination.test.ts
index a2d81ab5736..59f42be940c 100644
--- a/src/utilities/policies/__tests__/relayStylePagination.test.ts
+++ b/src/utilities/policies/__tests__/relayStylePagination.test.ts
@@ -1,9 +1,116 @@
-import { FieldFunctionOptions, InMemoryCache, isReference, makeReference } from '../../../core';
+import { FieldFunctionOptions, InMemoryCache, isReference, makeReference, StoreObject } from '../../../cache';
 import { relayStylePagination, TRelayPageInfo } from '../pagination';
 
 describe('relayStylePagination', () => {
   const policy = relayStylePagination();
 
+  describe('read', () => {
+    const fakeEdges = [
+      { node: { __ref: "A" }, cursor: "cursorA" },
+      { node: { __ref: "B" }, cursor: "cursorB" },
+      { node: { __ref: "C" }, cursor: "cursorC" },
+    ];
+
+    const fakeReadOptions = {
+      canRead() { return true },
+      readField(key, obj: StoreObject) {
+        return obj && obj[key];
+      },
+    } as FieldFunctionOptions;
+
+    it("should prefer existing.pageInfo.startCursor", () => {
+      const resultWithStartCursor = policy.read!({
+        edges: fakeEdges,
+        pageInfo: {
+          startCursor: "preferredStartCursor",
+          hasPreviousPage: false,
+          hasNextPage: true,
+        } as TRelayPageInfo,
+      }, fakeReadOptions);
+
+      expect(
+        resultWithStartCursor &&
+        resultWithStartCursor.pageInfo
+      ).toEqual({
+        startCursor: "preferredStartCursor",
+        endCursor: "cursorC",
+        hasPreviousPage: false,
+        hasNextPage: true,
+      });
+    });
+
+    it("should prefer existing.pageInfo.endCursor", () => {
+      const resultWithEndCursor = policy.read!({
+        edges: fakeEdges,
+        pageInfo: {
+          endCursor: "preferredEndCursor",
+          hasPreviousPage: false,
+          hasNextPage: true,
+        } as TRelayPageInfo,
+      }, fakeReadOptions);
+
+      expect(
+        resultWithEndCursor &&
+        resultWithEndCursor.pageInfo
+      ).toEqual({
+        startCursor: "cursorA",
+        endCursor: "preferredEndCursor",
+        hasPreviousPage: false,
+        hasNextPage: true,
+      });
+    });
+
+    it("should prefer existing.pageInfo.{start,end}Cursor", () => {
+      const resultWithEndCursor = policy.read!({
+        edges: fakeEdges,
+        pageInfo: {
+          startCursor: "preferredStartCursor",
+          endCursor: "preferredEndCursor",
+          hasPreviousPage: false,
+          hasNextPage: true,
+        },
+      }, fakeReadOptions);
+
+      expect(
+        resultWithEndCursor &&
+        resultWithEndCursor.pageInfo
+      ).toEqual({
+        startCursor: "preferredStartCursor",
+        endCursor: "preferredEndCursor",
+        hasPreviousPage: false,
+        hasNextPage: true,
+      });
+    });
+
+    it("should override pageInfo.{start,end}Cursor if empty strings", () => {
+      const resultWithEndCursor = policy.read!({
+        edges: [
+          { node: { __ref: "A" }, cursor: "" },
+          { node: { __ref: "B" }, cursor: "cursorB" },
+          { node: { __ref: "C" }, cursor: "" },
+          { node: { __ref: "D" }, cursor: "cursorD" },
+          { node: { __ref: "E" } },
+        ],
+        pageInfo: {
+          startCursor: "",
+          endCursor: "",
+          hasPreviousPage: false,
+          hasNextPage: true,
+        },
+      }, fakeReadOptions);
+
+      expect(
+        resultWithEndCursor &&
+        resultWithEndCursor.pageInfo
+      ).toEqual({
+        startCursor: "cursorB",
+        endCursor: "cursorD",
+        hasPreviousPage: false,
+        hasNextPage: true,
+      });
+    });
+  });
+
   describe('merge', () => {
     const merge = policy.merge;
     // The merge function should exist, make TS aware
diff --git a/src/utilities/policies/pagination.ts b/src/utilities/policies/pagination.ts
index fc885888905..525a2f5a7de 100644
--- a/src/utilities/policies/pagination.ts
+++ b/src/utilities/policies/pagination.ts
@@ -98,20 +98,29 @@ export function relayStylePagination<TNode = Reference>(
       if (!existing) return;
 
       const edges: TRelayEdge<TNode>[] = [];
-      let startCursor = "";
-      let endCursor = "";
+      let firstEdgeCursor = "";
+      let lastEdgeCursor = "";
       existing.edges.forEach(edge => {
         // Edges themselves could be Reference objects, so it's important
         // to use readField to access the edge.edge.node property.
         if (canRead(readField("node", edge))) {
           edges.push(edge);
           if (edge.cursor) {
-            startCursor = startCursor || edge.cursor;
-            endCursor = edge.cursor;
+            firstEdgeCursor = firstEdgeCursor || edge.cursor || "";
+            lastEdgeCursor = edge.cursor || lastEdgeCursor;
           }
         }
       });
 
+      let {
+        startCursor,
+        endCursor,
+      } = existing.pageInfo || {};
+      // If existing.pageInfo.{start,end}Cursor are undefined or "", default to
+      // firstEdgeCursor and/or lastEdgeCursor.
+      startCursor = startCursor || firstEdgeCursor;
+      endCursor = endCursor || lastEdgeCursor;
+
       return {
         // Some implementations return additional Connection fields, such
         // as existing.totalCount. These fields are saved by the merge