Getting all documents in collection in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Getting all documents in collection' mean in Firebase?

It means fetching every document stored inside a specific collection in Firestore, like getting all items from a folder.

beginner

Which Firebase function is used to get all documents from a collection?

The function getDocs() is used along with a collection reference to retrieve all documents.

beginner

How do you create a reference to a collection in Firebase Firestore?

Use collection(db, 'collectionName') where db is your Firestore instance and 'collectionName' is the name of your collection.

intermediate

What type of data structure do you get after fetching all documents from a collection?

You get a QuerySnapshot which contains multiple DocumentSnapshot objects, each representing a document.

intermediate

How can you access the data inside each document after fetching all documents?

You loop through the QuerySnapshot using forEach and call doc.data() on each DocumentSnapshot to get the document's data.

Which function fetches all documents from a Firestore collection?

A getDocs()

B fetchAll()

C getCollection()

D retrieveDocs()

The correct function to fetch all documents is getDocs().

What does collection(db, 'users') do?

A Updates all documents in 'users'

B Creates a new document in 'users'

C Deletes the 'users' collection

D References the 'users' collection in Firestore

It creates a reference to the 'users' collection in Firestore.

After fetching documents, which method gets the data inside a document?

A doc.getData()

B doc.data()

C doc.fetch()

D doc.read()

Use doc.data() to access the document's data.

What type of object holds all documents fetched from a collection?

A CollectionReference

B DocumentSnapshot

C QuerySnapshot

D DocumentReference

A QuerySnapshot contains all the documents fetched.

Which method is commonly used to loop through all documents in a QuerySnapshot?

A forEach()

B reduce()

C filter()

D map()

Use forEach() to loop through each document.

Explain step-by-step how to get all documents from a Firestore collection.

Think about how you open a folder, look at each file, and read its content.

What is the difference between a CollectionReference and a QuerySnapshot in Firestore?

Imagine a folder and the list of files inside it.

Practice

1. What does the getDocs function do in Firebase when used with a collection reference?

easy

A. It creates a new document in the collection.

B. It deletes all documents in the collection.

C. It updates all documents in the collection.

D. It fetches all documents from the specified collection.

Solution

1. Step 1: Understand the purpose of getDocs

   The getDocs function is designed to retrieve documents from a collection in Firebase.

2. Step 2: Identify the correct action

   Using getDocs with a collection reference returns all documents inside that collection.

3. Final Answer:

   It fetches all documents from the specified collection. -> Option D

4. Quick Check:

   getDocs fetches documents = A [OK]

Hint: Remember: getDocs reads all documents in a collection [OK]

Common Mistakes:

• Confusing getDocs with delete or update functions
• Thinking getDocs creates documents
• Assuming getDocs fetches a single document

2. Which of the following is the correct syntax to get all documents from a collection named users in Firebase?

easy

A. const querySnapshot = await getDocs(collection(db, 'users'));

B. const querySnapshot = getDocs('users');

C. const querySnapshot = await getDocs('db.users');

D. const querySnapshot = collection(db, 'users').getDocs();

Solution

1. Step 1: Identify correct function usage

   The getDocs function requires a collection reference, which is created by collection(db, 'users').

2. Step 2: Check for correct async usage

   Since getDocs returns a promise, it should be awaited with await.

3. Final Answer:

   const querySnapshot = await getDocs(collection(db, 'users')); -> Option A

4. Quick Check:

   Correct syntax uses await and collection() [OK]

Hint: Use await getDocs(collection(db, 'collectionName')) [OK]

Common Mistakes:

• Passing collection name as string directly to getDocs
• Calling getDocs as a method on collection()
• Missing await for async call

3. Given the following code snippet, what will be logged to the console?

const querySnapshot = await getDocs(collection(db, 'products'));
querySnapshot.forEach((doc) => {
  console.log(doc.id, doc.data());
});

medium

A. Nothing, because getDocs does not return documents.

B. Only the IDs of documents in the 'products' collection.

C. Each document's ID and its data object from the 'products' collection.

D. An error because forEach is not a function on querySnapshot.

Solution

1. Step 1: Understand getDocs return value

   The getDocs function returns a QuerySnapshot containing documents.

2. Step 2: Loop through documents with forEach

   The forEach method iterates over each document snapshot, allowing access to doc.id and doc.data().

3. Final Answer:

   Each document's ID and its data object from the 'products' collection. -> Option C

4. Quick Check:

   forEach logs id and data = D [OK]

Hint: Use forEach on querySnapshot to access each document [OK]

Common Mistakes:

• Thinking forEach is not available on querySnapshot
• Expecting only IDs without data
• Assuming getDocs returns a single document

4. What is wrong with this code snippet that tries to get all documents from the orders collection?

const ordersRef = collection(db, 'orders');
const snapshot = getDocs(ordersRef);
snapshot.forEach(doc => console.log(doc.id));

medium

A. Missing await before getDocs, so snapshot is a Promise, not data.

B. Using collection() incorrectly; it should be collection('orders', db).

C. forEach is not a valid method on snapshot.

D. doc.id does not exist on document snapshots.

Solution

1. Step 1: Check async usage of getDocs

   The getDocs function returns a Promise, so it must be awaited to get the snapshot.

2. Step 2: Understand consequences of missing await

   Without await, snapshot is a Promise object, which does not have a forEach method, causing an error.

3. Final Answer:

   Missing await before getDocs, so snapshot is a Promise, not data. -> Option A

4. Quick Check:

   Always await getDocs to get data [OK]

Hint: Always await getDocs to get actual documents [OK]

Common Mistakes:

• Forgetting to await getDocs
• Incorrect order of parameters in collection()
• Assuming forEach works on Promise

5. You want to get all documents from the tasks collection but only log the documents where the field completed is true. Which code snippet correctly does this?

hard

A. const querySnapshot = await getDocs(collection(db, 'tasks'));
querySnapshot.forEach(doc => {
if (doc.completed === true) {
console.log(doc.id, doc.data());
}
});

B. const querySnapshot = await getDocs(collection(db, 'tasks'));
querySnapshot.forEach(doc => {
if (doc.data().completed === true) {
console.log(doc.id, doc.data());
}
});

C. const querySnapshot = await getDocs(collection(db, 'tasks'));
querySnapshot.forEach(doc => {
if (doc.data.completed) {
console.log(doc.id, doc.data());
}
});

D. const querySnapshot = await getDocs(collection(db, 'tasks'));
querySnapshot.forEach(doc => {
if (doc.data('completed') === true) {
console.log(doc.id, doc.data());
}
});

Solution

1. Step 1: Access document data correctly

   Use doc.data() to get the document's data object, then check the completed field.

2. Step 2: Filter documents by completed field

   Check if doc.data().completed === true before logging the document.

3. Final Answer:

   Code snippet that checks doc.data().completed === true and logs matching docs. -> Option B

4. Quick Check:

   Use doc.data() to access fields [OK]

Hint: Check fields inside doc.data() for filtering [OK]

Common Mistakes:

• Trying to access fields directly on doc instead of doc.data()
• Using incorrect syntax like doc.data.completed
• Passing field name to doc.data()