Example of how to perform a get, add and update using Cloud Firestore in a Function

function.js

// Make sure to use firebase-admin and to initialize the app
const admin = require('firebase-admin')
admin.initializeApp(functions.config().firebase)

// Example of how I'm GETTING all the records in a collection AND using 
// the ID from the path of the document that triggered this function
exports.getLessonsAsync = functions.firestore
  .document('organizations/{organizationId}/lessons/{lessonId}')
  .onWrite(async function(change, context) {
    const result = await admin
      .firestore()
      .collection('organizations')
      .doc(context.params.organizationId)
      .collection('lessons')
      .get()

    result.forEach((doc) => {
      console.log(doc.id, '=>', doc.data())
    })
  })

// Example of how I'm getting ADDING a record to the collection AND using 
// the ID from the path of the document that triggered this function
exports.addLessonAsync = functions.firestore
  .document('organizations/{organizationId}/lessons/{lessonId}')
  .onWrite(async function(change, context) {
    await admin
      .firestore()
      .collection('organizations')
      .doc(context.params.organizationId)
      .collection('lessons')
      .add({
        name: 'Rock band project'
      })
  })

// Example of how I'm getting UPDATING a record to the collection AND using 
// the ID from the path of the document that triggered this function
  exports.updateLessonAsync = functions.firestore
  .document('organizations/{organizationId}/lessons/{lessonId}')
  .onWrite(async function(change, context) {
    await admin
      .firestore()
      .collection('organizations')
      .doc(context.params.organizationId)
      .collection('lessons')
      .doc(context.params.lessonId)
      .update({
        name: 'Rock band project v2'
      })
  })

// Example of how I'm determining what actually happened to a document
// by using the before and after data methods
exports.howToCheckStatusOfRecordUsingOnWrite = functions.firestore
  .document('organizations/{organizationId}/lessons/{lessonId}')
  .onWrite(async function(change, context) {
    // With onWrite you get the before and after so you can figure out if it was added, updated or deleted
    const originalLesson = change.before.data()
    const updatedLesson = change.after.data()

    if (!originalLesson) {
      console.log('New lesson so ignore')
    } else if (!updatedLesson) {
      console.log('Deleted lesson')
    } else {
      console.log('Updated lesson')
    }
  })