Versioned documents with Firestore Cloud Functions

index.es6.js

import * as admin from 'firebase-admin'
import * as functions from 'firebase-functions'

admin.initializeApp(functions.config().firebase)
const db = admin.firestore()

async function writeIntegerVersion(event) {
  const ref = event.data.ref
  if (ref.path.indexOf('/_versions/') >= 0) {
    return false
  }

  const snapshot = event.data
  const previous = event.data.previous
  // Return early if a previous snapshot existed AND its version differs than the current version.
  // FIXME: if a client modifies _version along with other fields, we won’t create a versioned snapshot.
  if (previous && previous.get('_version') != snapshot.get('_version')) {
    return false
  }

  let latest = 0
  const query = db.collection(`${ref.path}/_versions`).orderBy('_version', 'desc').limit(1)
  const querySnapshot = await query.get()
  if (!querySnapshot.empty) {
    latest = querySnapshot.docs[0].get('_version')
    console.log(`Highest version found: ${latest} @ ${querySnapshot.docs[0].ref.path}`)
    latest = +latest || 0
  }

  const version = latest + 1
  const data = event.data.data()
  data._version = version

  return Promise.all([
    ref.set({ _version: version }, { merge: true }),
    db.doc(`${ref.path}/_versions/${version}`).create(data)
  ])
}

async function writeTimestampVersion(event) {
  if (event.data.ref.path.indexOf('/_versions/') >= 0) {
    return false
  }
  return db.doc(`${event.data.ref.path}/_versions/${event.data.updateTime}`).create(event.data.data())
}

const ffd = functions.firestore.document

// Set fn to writeIntegerVersion for linear version numbers (more expensive)
// Set fn to writeTimestampVersion for timestamp version IDs (cheaper; no reads or updates)
const fn = writeTimestampVersion

// Root-level collection
export const createVersion = ffd('{collection}/{id}').onCreate(fn)
export const updateVersion = ffd('{collection}/{id}').onUpdate(fn)

// Subcollection nested 1-deep
export const createVersionN1 = ffd('{parent}/{pid}/{collection}/{id}').onCreate(fn)
export const updateVersionN1 = ffd('{parent}/{pid}/{collection}/{id}').onUpdate(fn)

// Subcollection nested 2-deep
export const createVersionN2 = ffd('{grandparent}/{gid}/{parent}/{pid}/{collection}/{id}').onCreate(fn)
export const updateVersionN2 = ffd('{grandparent}/{gid}/{parent}/{pid}/{collection}/{id}').onUpdate(fn)