title: TypeError: admin.firestore(...).collection(...).doc(...).collection(...).doc(...).get.then is not a function
body: <p>I'm trying to code a function that triggers when a user gets a new comment and sends a notification.Comments are stored in <code>/Users/{userID}/Notifications/{notificationID}</code>. Users save their device notification tokens to <code>/users/{userID}</code></p>
  
  <p>This is the whole <code>index.js</code> file</p>
  
  <pre><code>'use strict';
  
  const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  admin.initializeApp();
  
  exports.sendNotification = functions.firestore.document('Users/{userID}/Notifications/{notificationID}').onWrite((data, context) =&gt; {
  
      const to_user_id = context.params.userID;
      const notification_id = context.params.notificationID;
  
  
      return admin.firestore().collection('Users').doc(to_user_id).collection('Notifications').doc(notification_id).get.then(queryResult =&gt; {
  
          const from_user_id = queryResult.data().commentUID;
          const from_user_data = admin.firestore().collection('Users').doc(from_user_id).get();
          const to_user_data = admin.firestore().collection('Users').doc(to_user_id).get();
  
          return Promise.all([from_user_data, to_user_data]).then(result =&gt; {
  
              const from_name = result[0].data().name;
              const to_name = result[1].data().name;
  
              return console.log("FROM: " + from_name + " TO: " + to_name);
  
          });
  
      });
  
  });
  </code></pre>
  
  <p>And here's the <code>package.json</code> file. Everything is up-to-date</p>
  
  <pre><code>{
    "name": "functions",
    "description": "Cloud Functions for Firebase",
    "scripts": {
      "lint": "eslint .",
      "serve": "firebase serve --only functions",
      "shell": "firebase experimental:functions:shell",
      "start": "npm run shell",
      "deploy": "firebase deploy --only functions",
      "logs": "firebase functions:log"
    },
    "dependencies": {
      "firebase-admin": "~5.11.0",
      "firebase-functions": "^1.0.0"
    },
    "devDependencies": {
      "eslint": "^4.12.0",
      "eslint-plugin-promise": "^3.6.0"
    },
    "private": true
  }
  </code></pre>
  
  <p>Firebase gives the following error:</p>
  
  <pre><code>TypeError: admin.firestore(...).collection(...).doc(...).collection(...).doc(...).get.then is not a function
          at exports.sendNotification.functions.firestore.document.onWrite (/user_code/index.js:19:119)
          at Object.&lt;anonymous&gt; (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:112:27)
          at next (native)
          at /user_code/node_modules/firebase-functions/lib/cloud-functions.js:28:71
          at __awaiter (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:24:12)
          at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:82:36)
          at /var/tmp/worker/worker.js:700:26
          at process._tickDomainCallback (internal/process/next_tick.js:135:7)
  </code></pre>