Adding documents (add vs set) in Firebase - CLI Comparison

Introduction

When you want to save information in Firebase Firestore, you can add new documents in two main ways: using add or set. Both save data but work a bit differently on how they create or update documents.

When you want Firebase to create a new document with a unique ID automatically.

When you want to specify the exact ID for the document you are saving.

When you want to add a new document without worrying about overwriting existing data.

When you want to update or replace data in a document with a known ID.

When you want to merge new data into an existing document without deleting other fields.

Commands

This command adds a new document with a unique ID to the 'users' collection with the given data.

Terminal

firebase firestore:documents:add users '{"name":"Alice","age":30}'

Expected Output

Document added with ID: XyZ123AbC

This command sets the document with ID 'user123' in the 'users' collection to the given data, replacing any existing data.

Terminal

firebase firestore:documents:set users/user123 '{"name":"Bob","age":25}'

Expected Output

Document users/user123 successfully written.

This command updates the 'age' field in the 'user123' document without deleting other fields by merging the new data.

Terminal

firebase firestore:documents:set users/user123 '{"age":26}' --merge

Expected Output

Document users/user123 successfully written.

→

--merge - Merges new data with existing document fields instead of replacing.

This command lists documents in the 'users' collection so you can verify the data was added or updated.

Terminal

firebase firestore:documents:list users

Expected Output

Document ID: XyZ123AbC

  name: Alice

  age: 30

Document ID: user123

  name: Bob

  age: 26

Key Concept

If you remember nothing else from this pattern, remember: use add to create new documents with automatic IDs, and use set to create or update documents with specific IDs.

Common Mistakes

Using set without --merge when updating a document.

This replaces the entire document and deletes any fields not included in the new data.

Use set with the --merge flag to update only specific fields without deleting others.

Using add when you want to control the document ID.

Add always creates a new document with a random ID, so you cannot specify the ID.

Use set with the document ID path to specify the exact document to create or update.

Summary

Use 'add' to create new documents with unique IDs automatically generated by Firebase.

Use 'set' to create or overwrite documents with a specific ID you choose.

Use 'set' with the '--merge' flag to update parts of a document without deleting existing fields.

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