From 8f4ed8e10dd7b14c9b8e72e5eb8cc32bf10efbf6 Mon Sep 17 00:00:00 2001
From: Matt Dunn-Rankin <mattdunnrankin@khanacademy.org>
Date: Wed, 22 Apr 2020 12:00:52 -0700
Subject: [PATCH] add db snapshotting & better translation loader

---
 server/index.js      | 11 +++++++----
 server/index.test.js | 15 +--------------
 server/loaders.js    | 32 ++++++++++++++++++--------------
 server/package.json  |  1 +
 server/yarn.lock     |  5 +++++
 5 files changed, 32 insertions(+), 32 deletions(-)

diff --git a/server/index.js b/server/index.js
index 75a9a06..44176bc 100644
--- a/server/index.js
+++ b/server/index.js
@@ -1,7 +1,7 @@
 const { ApolloServer, gql } = require("apollo-server");
 
 const connectToDb = require("./db");
-const { loadItems, loadItemTranslation } = require("./loaders");
+const { loadItems, buildItemTranslationLoader } = require("./loaders");
 
 const typeDefs = gql`
   type Item {
@@ -16,8 +16,8 @@ const typeDefs = gql`
 
 const resolvers = {
   Item: {
-    name: async (item, _, { db }) => {
-      const translation = await loadItemTranslation(db, item.id, "en");
+    name: async (item, _, { itemTranslationLoader }) => {
+      const translation = await itemTranslationLoader.load(item.id);
       return translation.name;
     },
   },
@@ -31,7 +31,10 @@ const server = new ApolloServer({
   resolvers,
   context: async () => {
     const db = await connectToDb();
-    return { db };
+    return {
+      db,
+      itemTranslationLoader: buildItemTranslationLoader(db),
+    };
   },
 });
 
diff --git a/server/index.test.js b/server/index.test.js
index 9883152..82fd4cc 100644
--- a/server/index.test.js
+++ b/server/index.test.js
@@ -72,24 +72,11 @@ it("can load items", async () => {
         ],
       ],
       Array [
-        "SELECT * FROM item_translations WHERE item_id = ? AND locale = ? LIMIT 1",
+        "SELECT * FROM item_translations WHERE item_id IN (?,?,?) AND locale = \\"en\\"",
         Array [
           38911,
-          "en",
-        ],
-      ],
-      Array [
-        "SELECT * FROM item_translations WHERE item_id = ? AND locale = ? LIMIT 1",
-        Array [
           38912,
-          "en",
-        ],
-      ],
-      Array [
-        "SELECT * FROM item_translations WHERE item_id = ? AND locale = ? LIMIT 1",
-        Array [
           38913,
-          "en",
         ],
       ],
     ]
diff --git a/server/loaders.js b/server/loaders.js
index bfd0d81..bf1c026 100644
--- a/server/loaders.js
+++ b/server/loaders.js
@@ -1,3 +1,5 @@
+const DataLoader = require("dataloader");
+
 async function loadItems(db, ids) {
   const qs = ids.map((_) => "?").join(",");
   const [rows, _] = await db.execute(
@@ -8,19 +10,21 @@ async function loadItems(db, ids) {
   return rows;
 }
 
-async function loadItemTranslation(db, itemId, locale) {
-  const [
-    rows,
-    _,
-  ] = await db.execute(
-    `SELECT * FROM item_translations WHERE item_id = ? AND locale = ? LIMIT 1`,
-    [itemId, locale]
-  );
-  if (rows.length === 0) {
-    throw new Error(`could not load translation for ${itemId}, ${locale}`);
-  }
+const buildItemTranslationLoader = (db) =>
+  new DataLoader(async (itemIds) => {
+    const qs = itemIds.map((_) => "?").join(",");
+    const [rows, _] = await db.execute(
+      `SELECT * FROM item_translations WHERE item_id IN (${qs}) AND locale = "en"`,
+      itemIds
+    );
 
-  return rows[0];
-}
+    const rowsByItemId = new Map(rows.map((row) => [row.item_id, row]));
 
-module.exports = { loadItems, loadItemTranslation };
+    return itemIds.map(
+      (itemId) =>
+        rowsByItemId.get(itemId) ||
+        new Error(`could not find translation for item ${itemId}`)
+    );
+  });
+
+module.exports = { loadItems, buildItemTranslationLoader };
diff --git a/server/package.json b/server/package.json
index 2b128fe..c9a477c 100644
--- a/server/package.json
+++ b/server/package.json
@@ -6,6 +6,7 @@
   "license": "MIT",
   "dependencies": {
     "apollo-server": "^2.12.0",
+    "dataloader": "^2.0.0",
     "dotenv": "^8.2.0",
     "graphql": "^15.0.0",
     "mysql2": "^2.1.0"
diff --git a/server/yarn.lock b/server/yarn.lock
index 3d2b94d..dbd8503 100644
--- a/server/yarn.lock
+++ b/server/yarn.lock
@@ -1686,6 +1686,11 @@ data-urls@^1.1.0:
     whatwg-mimetype "^2.2.0"
     whatwg-url "^7.0.0"
 
+dataloader@^2.0.0:
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/dataloader/-/dataloader-2.0.0.tgz#41eaf123db115987e21ca93c005cd7753c55fe6f"
+  integrity sha512-YzhyDAwA4TaQIhM5go+vCLmU0UikghC/t9DTQYZR2M/UvZ1MdOhPezSDZcjj9uqQJOMqjLcpWtyW2iNINdlatQ==
+
 debug@2.6.9, debug@^2.2.0, debug@^2.3.3:
   version "2.6.9"
   resolved "https://registry.yarnpkg.com/debug/-/debug-2.6.9.tgz#5d128515df134ff327e90a4c93f4e077a536341f"