Creating collections and documents in Firebase - Practice Exercises

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when you add a document without specifying an ID?

In Firebase Firestore, if you add a document to a collection without specifying an ID, what will be the result?

Firebase

db.collection('users').add({name: 'Alice', age: 30});

A Firestore automatically generates a unique ID for the new document.

B The document is added with an empty string as its ID.

C The operation fails with an error because ID is required.

D The document overwrites the first document in the collection.

💡 Hint

Think about how Firestore handles document IDs when none are provided.

When you use add() without an ID, Firestore creates a unique ID automatically to avoid conflicts.

❓ Configuration

intermediate

Which code snippet correctly creates a collection and a document with a custom ID?

Choose the code that creates a collection named products and adds a document with ID prod123 containing a field price set to 100.

A db.collection('products').doc().set({id: 'prod123', price: 100});

B db.collection('products').add({id: 'prod123', price: 100});

C db.collection('products').doc('prod123').set({price: 100});

D db.doc('products/prod123').add({price: 100});

💡 Hint

Remember that doc() with an argument sets the document ID explicitly.

Using doc('prod123') sets the document ID to prod123. Then set() adds the data.

❓ Architecture

advanced

How does Firestore organize data when you create nested collections?

Consider you have a collection users, each user document has a subcollection orders. How is this data organized in Firestore?

A Subcollections are separate collections linked by path, not nested inside documents.

B Each <code>orders</code> subcollection is stored inside its parent user document as a nested object.

C All subcollections are merged into the main <code>users</code> collection.

D Firestore duplicates subcollection data inside the parent document for faster access.

💡 Hint

Think about how Firestore paths work for collections and documents.

Firestore stores subcollections as separate collections identified by their path under a document, not nested inside the document data.

❓ security

advanced

Which Firestore security rule allows only authenticated users to create documents in the 'messages' collection?

Choose the correct Firestore security rule snippet that permits only signed-in users to add documents to the messages collection.

A match /messages/{msgId} { allow create: if request.auth == null; }

B match /messages/{msgId} { allow create: if request.auth != null; }

C match /messages/{msgId} { allow create: if true; }

D match /messages/{msgId} { allow create: if request.auth.uid == ''; }

💡 Hint

Check how to verify if a user is signed in using request.auth.

The condition request.auth != null ensures the user is authenticated before allowing create.

✅ Best Practice

expert

What is the best practice for structuring Firestore collections to optimize read performance for user profiles and their posts?

You want to design Firestore data for users and their posts. Which approach optimizes read performance and scalability?

A Duplicate posts data inside both user documents and a posts collection.

B Store all posts as a subcollection under each user document.

C Embed all posts inside the user document as an array.

D Store all posts in a single top-level collection with a userId field to filter posts.

💡 Hint

Consider Firestore limits on document size and query efficiency.

Storing posts in a top-level collection with a userId field allows efficient queries and avoids document size limits.

Practice

1. What is a collection in Firebase Firestore?

easy

A. A function to add data.

B. A single document storing data.

C. A group that holds multiple documents.

D. A method to delete documents.

Solution

1. Step 1: Understand Firestore structure

   Firestore organizes data in collections and documents.

2. Step 2: Define collection role

   A collection groups multiple documents together.

3. Final Answer:

   A group that holds multiple documents. -> Option C

4. Quick Check:

   Collection = group of documents [OK]

Hint: Collections hold documents, not single data items. [OK]

Common Mistakes:

• Confusing collection with a single document
• Thinking collection is a method
• Mixing collection with data fields

2. Which Firebase Firestore method creates a document with an auto-generated ID?

easy

A. set()

B. add()

C. update()

D. get()

Solution

1. Step 1: Recall document creation methods

   Firestore uses set() to create or overwrite documents with specific IDs.

2. Step 2: Identify method for auto-generated IDs

   The add() method creates a document with an auto-generated ID inside a collection.

3. Final Answer:

   add() -> Option B

4. Quick Check:

   Auto-ID document creation = add() [OK]

Hint: Use add() for auto-generated document IDs. [OK]

Common Mistakes:

• Using set() and expecting auto-ID
• Confusing update() with creation
• Using get() which only reads data

3. What will happen when this code runs?

db.collection('users').doc('user1').set({name: 'Anna', age: 25});

medium

A. Creates or overwrites 'user1' document with name and age.

B. Adds a new document with auto-generated ID.

C. Updates only the age field in 'user1' document.

D. Throws an error because set() needs add() instead.

Solution

1. Step 1: Analyze method usage

   set() on a doc reference creates or overwrites that document.

2. Step 2: Check document ID and data

   The doc ID is 'user1', so this document will be created or replaced with given data.

3. Final Answer:

   Creates or overwrites 'user1' document with name and age. -> Option A

4. Quick Check:

   set() on doc('user1') = overwrite/create [OK]

Hint: set() replaces document at given ID, add() creates new ID. [OK]

Common Mistakes:

• Thinking add() is needed for specific IDs
• Assuming update() is called by set()
• Confusing set() with partial update

4. Identify the error in this Firestore code:

db.collection('products').add({name: 'Book', price: 10}).doc('p1').set({price: 12});

medium

A. No error, code works fine.

B. set() cannot update price field.

C. Collection name 'products' is invalid.

D. add() returns a promise, chaining doc() is invalid.

Solution

1. Step 1: Understand add() return value

   add() returns a promise resolving to a document reference, not a direct reference.

2. Step 2: Check chaining doc() after add()

   Calling doc() on the promise or document reference from add() is invalid syntax.

3. Final Answer:

   add() returns a promise, chaining doc() is invalid. -> Option D

4. Quick Check:

   add() returns promise, cannot chain doc() directly [OK]

Hint: add() returns promise; await before doc() calls. [OK]

Common Mistakes:

• Chaining doc() immediately after add()
• Ignoring promise returned by add()
• Assuming set() cannot update fields

5. You want to create a collection 'orders' and add a document with a specific ID 'order123' containing {item: 'Pen', quantity: 3}. Which code snippet is correct?

hard

A. db.collection('orders').doc('order123').set({item: 'Pen', quantity: 3});

B. db.collection('orders').add({item: 'Pen', quantity: 3, id: 'order123'});

C. db.collection('orders').doc().set({item: 'Pen', quantity: 3, id: 'order123'});

D. db.collection('orders').set('order123', {item: 'Pen', quantity: 3});

Solution

1. Step 1: Identify method to create document with specific ID

   Use doc('order123') to specify the document ID explicitly.

2. Step 2: Use set() to create or overwrite document data

   set() writes the data to the specified document ID.

3. Final Answer:

   db.collection('orders').doc('order123').set({item: 'Pen', quantity: 3}); -> Option A

4. Quick Check:

   Specific ID document creation = doc(id).set() [OK]

Hint: Use doc('id').set() for specific document IDs. [OK]

Common Mistakes:

• Using add() with id field (does not set document ID)
• Calling doc() without ID for specific ID
• Using set() directly on collection