How to subscribe a firebase function so that it executes via a pub/sub trigger - firebase

Question

I have been asked to create a firebase (fb) function that triggers onUpdate. Then I have to gather a bunch of information from the fb database and publish a message so that another function triggers at that point.
fb update triggers functionA
functionA publishes a message
functionB is a subscriber to that topic and is triggered after the message publishes.
I have the basics of the onUpdate trigger below:
const functions = require("firebase-functions"),
Promise = require("promise"),
PubSub = require(`#google-cloud/pubsub`),
admin = require("firebase-admin");
const pubsub = new PubSub();
exports.checkInOrder = functions.database
.ref("/orders/{id}")
.onUpdate((change, context) => {
const after = change.after.val();
// check the status: "pending-pickup" or "fulfilled" TODO
if (after.status === "new") {
console.log("ended because package is new.");
return null;
}
let dsObj = {};
const orderId = context.params.id;
const topicName = 'check-in-order';
const subscriptionName = 'check-in-order';
return // how would I send the message to the pubsub here?
});
So to summarize:
How do I send a message to pubsub
How do I subscribe a firebase function to trigger when a topic receives a message?
If it sounds very confusing I'm sorry - I am completely lost here. Thanks!

Answer 1

So I finally figured it out. Pretty straight forward, just felt overwhelmed with learning all of this stuff at once and in a rushed time frame. Below is my code. I included the whole page so in case this might help someone else out there in the future.
const functions = require("firebase-functions"),
Promise = require("promise"),
PubSub = require(`#google-cloud/pubsub`),
admin = require("firebase-admin");
const init = () => {
const topicName = "check-in-order";
pubsub
.createTopic(topicName)
.then(results => {
const topic = results[0];
console.log(`Topic ${topicName} created.`);
return;
})
.catch(err => {
console.error("ERROR on init:", err);
return;
});
};
const pubsub = new PubSub();
exports.orderCreated = functions.database
.ref("/orders/{id}")
.onUpdate((change, context) => {
console.log("it's happening!");
const after = change.after.val();
console.log("after::::>", after)
if (after.status === "new") {
console.log('stopped because status is new');
return null;
}
if (after.status === "pending-pickup" || after.status === "fulfilled") {
const orderId = context.params.id;
const topicName = "check-in-order";
let { assignedSlot, userId, parcelLockerId, carrier, trackingNumber, orderInDate, pickUpCode, status, } = after;
const dsObj = {
order: {
orderId,
userId,
parcelLockerId,
carrier,
trackingNumber,
orderInDate,
pickUpCode,
status,
}
};
const dataBuffer = new Buffer.from(dsObj.toString());
// publish to trigger check in function
return pubsub
.topic(topicName)
.publisher()
.publish(dataBuffer)
.then(messageId => {
console.log(`:::::::: Message ${messageId} has now published. :::::::::::`);
return true;
})
.catch(err => {
console.error("ERROR:", err);
throw err;
});
}
return false;
});
exports.checkInOrder = () => {
}
exports.checkIn = functions.pubsub.topic('check-in-order').onPublish((message) => {
console.log("everything is running now", message);
return true;
});
init();