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Getting values from single Firebase Firestore Document

This is definitely a newbie question, and in part answered in the Firebase documentation, but for the life of me it's not working when implementing it in my own code - so I'm hoping the community can help me understand what I am doing wrong, and how to fix it.
When getting documents from Firestore, I can't access the actual values within, due to its structure, so when setting e.g. "var name = doc.name" it just gives me undefined. For getting MULTIPLE documents, I've already found apiece of code that works:
// Getting the document
docRef.collection(collectionRef).get()
.then((snapshots) => cleanData(snapshots))
.then((items) => items.map((item) => sampleFunction(item)));
// Firebase Utility cleaning documents (array)
function cleanData(snapshots) {
let data = [];
snapshots.forEach(function(doc) {
data.push({
id: doc.id,
...doc.data()
});
});
return data;
}
But when using this piece of code with e.g. collection("x").doc("id"), then it throws the error:
"Uncaught (in promise) TypeError: snapshots.forEach is not a function"
So I went ahead to modify the function as follows:
// Firebase Utility cleaning document (single)
function cleanDoc(snap) {
let data = [];
data.push({
id: doc.id,
...doc.data()
});
return data;
}
But that gives me "undefined" when attempting to access the values in my function again...
The documentation (in the city example) says to define a class. When I did that, I was able to get values from one document, but it gave me undefined the second time I called the same function on one page.
For context, I'm trying to display a User Profile, which displays people they work with on a project, which means I call these profiles as well, the data structure just callsa reference to the "worked with" profiles, and I get their ID's just fine, but when attempting to render an HTML item for each, the values within their profiles are undefined....Its confusing the hell out of me anyways.

If your function is an async function:
collectionSnap = await docRef.collection(collectionRef).get();
val items=[]
await Promise.all(querySnap.docs.map(async (doc) => {
// Do your your work and populate items
}));
// Do your work with items
You can try this approach to processing your documents.

Not able to get key in nested data

I have created a realtime database in Firebase and have some data looking like this inside a key called "firms":
I got the user key and wants to find the key where the user is placed. I have tried something like this:
let userkey = 'the key that I have for the user'
ref.child('firms').child('users').orderByChild('user').equalTo('userkey').on("value",
function(snapshot) {
console.log(snapshot.val());
});
Is there something that i'm missing?

You have to do as follows:
ref
.child('firms')
.orderByChild('users/user')
.equalTo(userkey) // Warning: in your question you do equalTo('userkey') with quotes
.on('value', function (snapshot) {
console.log(Object.keys(snapshot.val())[0]);
});
As explained in the doc, you need to specify the relative path to the nested child in your orderByChild() call.

How to make array by checking object property in firebase?

I have a list of users in firebase database and I want to make an array of all users whose isDonor value is true. How can I accomplish that?

Something like this should work:
let ref = firebase.database().ref("users");
ref.orderByChild("isDonor").equalTo(true).once("value").then((results) => {
results.forEach((snapshot) => {
console.log(snapshot.key, snapshot.val());
});
});
Also see the Firebase documentation on sorting and filtering data.

Firebase orderByKey().startAt() not working as expected. What's wrong?

I'm trying to get firebase data from a node which uid must start with a passed string.
I tried a code but I always get the same data. The database data is as following:
And I'm using the following code:
var ref = firebase.database().ref("restaurantes/history");
ref.orderByKey().startAt(userUID).once("child_added", function(snapshot) {
snapshot.forEach(child => {
if(child.key == "orders")
{
console.log(child.val());
_.each(child.val(), (value, key) => {
arrtmp.push(value)
})
}
})
If user is "FKQLlqa" I should get the history data shown in the picture. If I user is "abc" I shouldn't get any data. But I always get the data shown in the picture. Should I use another way of querying? Or I should use a key field inside orders and payments data?
Regards!

Try the following:
var ref = firebase.database().ref("restaurantes/history");
ref.child(userUID).once("value", function(snapshot) {
if (snapshot.exists()) {
console.log(snapshot.val());
}
else {
console.log("different user");
});
This will check if the snapshot that contains the userId (added as a parameter in the child() method), already exists in the database then you will be able to retrieve the data under the userId.
For reference:
https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#exists

Peter's answer is the correct solution. I'm merely adding this for completeness.
When you call orderBy... on a Firebase reference, the database orders all child nodes on the key/value/child that you specify.
If you then subsequently call startAt(...) on the query, it finds the (first) node that starts with that value and starts returning all results from there. So if you start at FKQLlqa, it will start returning keys at FKQLlqa and then return all keys after it.
If you want to return the child node(s) with a specific key/value/child, you'd use equalTo(...). So:
ref.orderByKey().equalTo(userUID).once("child_added", function(snapshot) {
...
But as Peter said already, this is just a more expensive way to look up a child with a known key. I highly recommend using his better approach: ref.child(userUID).once("value".

How to push an array value in Firebase Firestore

I am trying to push an array element but am destroying all the content there and replacing with the pushed data:
db .collection('households')
.doc(householdId)
.set( { users: [uid], }, { merge: true }, )
.then(() => { resolve(); })
.catch(() => reject());
I thought the merge true doesn't destroy the data that is already there? Struggling a little with the firestore api docs.
This is the structure of my data:
households
2435djgnfk
users [
0: user1
1: user2
]
Thank you!

You should use Firestore Transaction for this.
const householdRef = db.collection('households').doc(householdId);
const newUid = '1234'; // whatever the uid is...
return db.runTransaction((t) => {
return t.get(householdRef).then((doc) => {
// doc doesn't exist; can't update
if (!doc.exists) return;
// update the users array after getting it from Firestore.
const newUserArray = doc.get('users').push(newUid);
t.set(householdRef, { users: newUserArray }, { merge: true });
});
}).catch(console.log);
Updating an array or a stored object without getting it first will always destroy the older values inside that array/object in firestore.
This is because they are fields and not actual document themselves. So, you have to first get the document and then update the value after that.

I think now you can do it better with the update command on document by using FieldValue.arrayUnion without destroying data that was added meanwhile. Like this:
const admin = require('firebase-admin');
let db = admin.firestore();
const FieldValue = admin.firestore.FieldValue;
let collectionRef = db.collection(collection);
let ref = collectionRef.doc(id);
let setWithOptions = ref.update(arrayFieldName, FieldValue.arrayUnion(value));
As described in https://firebase.googleblog.com/2018/08/better-arrays-in-cloud-firestore.html

Arrays in Firestore don't work like this. According to the documentation:
Although Cloud Firestore can store arrays, it does not support querying array members or updating single array elements.
If you want to change any element in an array, you have to read the array values from the document first, make changes to it in the client, then write the entire array back out.
There are probably other ways to model your data that are better for your use case. That page of documentation linked above has some solutions.