Firebase function - how to create multiple documents from different collections - node.js

Question

I am fairly new to writing functions and cannot figure out how to solve my issue or even search for it properly.
I have three collections:
current_projects
vendors
vendor_projects
when a new project is created I want the function to take all documents in vendors, add certain fields from them to vendor_projects, and include the project_id field that is created in current_projects.
Do I need a for loop to accomplish this or is there other syntax that could be utilized?
My current function is below. This creates on document using the new project_id field but doesnt take any of the fields from vendors. Any input is greatly appreciated.
exports.createProjVen = functions.firestore.document("/Current_projects/{id}")
.onCreate((snap, context)=>{
console.log(snap.data());
const id = snap.data().id;
// const collection = context.params.Current_projects;
// const id = context.params.id;
const projectVendors = admin.firestore().collection("project_vendors");
// const vendors = admin.firestore().collection("vendors");
return projectVendors.doc(id).set({
actual: "",
budget: "",
id: "23121",
project_id: id,
toggle: "true",
type: "Fixtures",
vendor_company: "tes",
vendor_contact: "tes",
vendor_email: "jj#j.com",
vendor_id: "test",
vendor_phone_number: "test"});
});
Adding more details:
When a new project is added it creates a record in current_projects.
I want the function to be able to query all documents in the vendor collection when a new record is created in current_projects.
From there, I want to grab the highlighted fields from the vendor documents, add the id from that was created from current_projects (highlighted in the first screen shot), and create a new document in project_vendors (third screen shot below) for every document in the vendors table.

Answer 1

If you are trying to created a document in project_vendors collection for every vendor in vendors after a project is created then you can map an array of promises and then use Promise.all() as shown below:
exports.createProjVen = functions.firestore.document("/Current_projects/{id}")
.onCreate((snap, context) => {
const docSnap = snap.data();
const id = context.params.id;
const vendorsSnap = await admin.firestore().collection("vendors").get();
const vendorsData = vendorsSnap.docs.map((d) => ({ id: d.id, ...d.data() }))
const promises = [];
const vendorPrsCol = admin.firestore().collection("project_vendors");
vendorsData.forEach((vendor) => {
const data = {
projectId: id,
email: vendor.email,
// other vendor fields
}
promises.push(vendorPrsCol.add(data));
})
await Promise.all(promises);
console.log("Vendor Projects added");
return null;
});