Creating collections and documents in Firebase - Quick Revision & Summary

Recall & Review

beginner

What is a collection in Firebase Firestore?

A collection is like a folder that holds multiple documents. It organizes data into groups for easy access and management.

beginner

How do you add a new document to a collection in Firebase Firestore?

You use the add() method on a collection reference to create a new document with an auto-generated ID, or doc().set() to specify the document ID.

intermediate

What is the difference between add() and set() when creating documents?

add() creates a document with a random ID inside a collection. set() writes data to a document with a specified ID, creating it if it doesn't exist.

beginner

Can a document exist without being inside a collection in Firestore?

No. Every document must be inside a collection. Collections hold documents, and documents can contain subcollections.

intermediate

What happens if you try to create a document with an ID that already exists?

Using set() will overwrite the existing document data unless you use merge options. Using add() will always create a new document with a unique ID.

In Firebase Firestore, what is the first step to add a new document?

A Create a new database

B Get a reference to a collection

C Write data directly without reference

D Delete an existing document

You must first get a reference to the collection where you want to add the document.

Which method automatically generates a document ID when adding data?

A set()

B update()

C add()

D get()

add() creates a new document with a random unique ID.

Can you create a document without specifying a collection in Firestore?

A No, documents must be inside collections

B Only for subcollections

C Only if you use the admin SDK

D Yes, documents can exist alone

Documents must always be inside collections in Firestore.

What does set() do if the document ID already exists?

A Overwrites the existing document data

B Deletes the document

C Creates a new document with a different ID

D Throws an error

set() overwrites the existing document unless merge options are used.

Which of these is true about Firestore collections?

A Collections can contain other collections directly

B Collections are the same as documents

C Documents can contain other documents

D Collections contain documents, which can have subcollections

Collections hold documents, and documents can have subcollections.

Explain how to create a new document inside a collection in Firebase Firestore.

Think about how folders and files work on your computer.

Describe the difference between the add() and set() methods when creating documents.

One is like dropping a file in a folder with a random name, the other is like naming the file yourself.

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