Add example for Cloud Firestore

examples/CloudFirestore.md

@@ -0,0 +1,34 @@
+# Using DataLoader with Cloud Firestore
+
+Cloud Firestore is a "NoSQL" document-oriented database which supports [in operator](https://firebase.google.com/docs/firestore/query-data/queries#in_not-in_and_array-contains-any),
+making it can be used with DataLoader.
+
+Here we build an example Cloud Firestore DataLoader using [firebase-admin](https://firebase.google.com/docs/admin/setup?hl=en).
+
+```js
+const admin = require("firebase-admin");
+
+admin.initializeApp({
+  // You need to Initialize the SDK
+});
+
+const datastoreLoader = new DataLoader(
+  async (keys) => {
+    const snapshot = await admin
+      .firestore()
+      .collection(`users`)
+      .where(admin.firestore.FieldPath.documentId(), `in`, keys)
+      .get();
+    // By default, a query retrieves all documents that satisfy the query in ascending order by document ID.
+    // Therefore, it is recommended to sort by key
+    return keys.map((key) => {
+      const doc = snapshot.docs.find((doc) => doc.id === key);
+      return doc ? { ...doc.data(), id: doc.id } : null;
+    });
+  },
+  {
+    // The in clause of Cloud Firestore is support up to 10 comparison values
+    maxBatchSize: 10,
+  }
+);
+```