Adding documents (add vs set) in Firebase - Hands-On Comparison

Adding Documents in Firebase: Using add() vs set()

📖 Scenario: You are building a simple app to store user profiles in Firebase Firestore. You want to learn how to add new user documents to a collection using two different methods: add() and set().This will help you understand how to create documents with automatic IDs and how to create or overwrite documents with specific IDs.

🎯 Goal: Build a Firebase Firestore setup where you first create a collection called users and add user documents using add() and set() methods correctly.

📋 What You'll Learn

Create a Firestore collection reference called usersRef pointing to the users collection

Add a new user document with automatic ID using add() with exact data

Create a user document with a specific ID using set() with exact data

Use correct Firestore syntax for both methods

💡 Why This Matters

🌍 Real World

Adding and managing user data in Firestore is common in apps like social networks, e-commerce, and content management systems.

💼 Career

Understanding Firestore document creation methods is essential for cloud developers working with Firebase backend services.

1

Create Firestore collection reference

Create a variable called usersRef that references the Firestore collection named users using firebase.firestore().collection('users').

Firebase

# Your code here

Hint

Use firebase.firestore().collection('users') to get the collection reference.

2

Add a user document with automatic ID using add()

Use the add() method on usersRef to add a new user document with these exact fields: { name: 'Alice', age: 30 }.

Firebase

const usersRef = firebase.firestore().collection('users');
// Your code here

Hint

Call usersRef.add() with the user data object.

3

Create a user document with a specific ID using set()

Create a document reference called bobDoc inside usersRef with the ID 'bob123' using usersRef.doc('bob123'). Then use set() on bobDoc to set the user data { name: 'Bob', age: 25 }.

Firebase

const usersRef = firebase.firestore().collection('users');
usersRef.add({ name: 'Alice', age: 30 });
// Your code here

Hint

Use usersRef.doc('bob123') to get the document reference, then call set() with the data.

4

Complete Firestore document additions

Ensure the full code includes the usersRef collection reference, the add() call for Alice, and the set() call for Bob with ID 'bob123'.

Firebase

const usersRef = firebase.firestore().collection('users');
usersRef.add({ name: 'Alice', age: 30 });
const bobDoc = usersRef.doc('bob123');
bobDoc.set({ name: 'Bob', age: 25 });
// Your code here

Hint

Review your code to confirm all parts are included.

Practice

1. What is the main difference between add and set when adding documents in Firebase?

easy

A. add creates a document with an automatic unique ID, set uses a specified ID.

B. add updates existing documents, set only creates new documents.

C. add deletes documents, set reads documents.

D. add requires a document ID, set generates one automatically.

Solution

1. Step 1: Understand add behavior

   add creates a new document with a unique ID generated by Firebase automatically.

2. Step 2: Understand set behavior

   set creates or updates a document at a specified ID you provide.

3. Final Answer:

   add creates a document with an automatic unique ID, set uses a specified ID. -> Option A

4. Quick Check:

   add auto ID, set specified ID = A [OK]

Hint: Remember: add auto-generates ID, set needs your ID [OK]

Common Mistakes:

• Thinking set always creates new documents only
• Confusing which method generates IDs automatically
• Assuming add can update existing documents

2. Which of the following is the correct syntax to add a new document with an automatic ID in Firebase Firestore?

easy

A. db.collection('users').add({name: 'Alice'})

B. db.collection('users').set({name: 'Alice'})

C. db.collection('users').doc('user1').add({name: 'Alice'})

D. db.collection('users').doc().add({name: 'Alice'})

Solution

1. Step 1: Identify method for automatic ID

   add creates a new document with an automatic ID inside a collection.

2. Step 2: Check syntax correctness

   db.collection('users').add({name: 'Alice'}) is the correct syntax to add a new document with auto ID.

3. Final Answer:

   db.collection('users').add({name: 'Alice'}) -> Option A

4. Quick Check:

   Use add for auto ID = B [OK]

Hint: Use add() on collection to auto-generate document ID [OK]

Common Mistakes:

• Using set() without specifying a document ID
• Calling add() on a document reference instead of collection
• Using doc() without an ID then calling add()

3. What will be the result of this code snippet?

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

medium

A. The document 'p1' will have {name: 'Book', price: 12}

B. The document 'p1' will have {name: 'Book', price: 10, price: 12}

C. The document 'p1' will have only {price: 12}, name is removed

D. An error occurs because set is called twice

Solution

1. Step 1: Analyze first set call

   The first set creates or overwrites document 'p1' with {name: 'Book', price: 10}.

2. Step 2: Analyze second set call without merge

   The second set overwrites the entire document with {price: 12}, removing 'name' because merge option is not used.

3. Final Answer:

   The document 'p1' will have only {price: 12}, name is removed -> Option C

4. Quick Check:

   set without merge overwrites whole document = C [OK]

Hint: set without merge replaces whole document, removing old fields [OK]

Common Mistakes:

• Assuming set merges fields by default
• Expecting both name and price to remain
• Thinking calling set twice causes error

4. Identify the error in this code snippet and how to fix it:

db.collection('orders').add({id: 'order1', total: 50});
db.collection('orders').doc('order1').set({total: 60});

medium

A. No error; both lines work fine together.

B. The first line creates a document with a random ID, so the second line updates a different document.

C. The second line should use add() instead of set().

D. The first line should specify the document ID using doc('order1').set() instead of add().

Solution

1. Step 1: Understand add() behavior

   The first line adds a document with a random ID, ignoring the 'id' field inside data.

2. Step 2: Understand doc('order1').set() behavior

   The second line sets data on a document with ID 'order1', which is different from the random ID created before.

3. Step 3: Identify mismatch

   Because IDs differ, the second line updates a different document than the one created by add().

4. Final Answer:

   The first line creates a document with a random ID, so the second line updates a different document. -> Option B

5. Quick Check:

   add() random ID ≠ doc('order1') ID = A [OK]

Hint: add() ignores id field; doc(id).set() targets specific document [OK]

Common Mistakes:

• Assuming add() uses the id field as document ID
• Using set() on a doc without knowing if it exists
• Expecting add() and set() to affect same document automatically

5. You want to update only the 'price' field of a document without removing other fields. Which Firebase method and option should you use?

hard

A. Use add() with the new price data.

B. Use set() without any options.

C. Use update() without any options.

D. Use set() with {merge: true} option.

Solution

1. Step 1: Understand add() behavior

   add() creates a new document with a random ID; it cannot update existing documents.

2. Step 2: Understand set() without merge

   set() without options overwrites the whole document, removing other fields.

3. Step 3: Understand set() with merge option

   set() with {merge: true} updates only specified fields, keeping others intact.

4. Step 4: Consider update()

   update() also updates fields but requires the document to exist; set() with merge works even if document is new.

5. Final Answer:

   Use set() with {merge: true} option. -> Option D

6. Quick Check:

   Partial update without overwrite = set() + merge true = D [OK]

Hint: Use set() with merge:true to update fields without deleting others [OK]

Common Mistakes:

• Using add() to update existing documents
• Using set() without merge and losing data
• Confusing update() with set() merge behavior