Adding documents (add vs set) in Firebase - Practice Questions

Challenge - 5 Problems

❓ service_behavior

intermediate

Difference in document ID generation between add() and set()

In Firebase Firestore, what happens when you use add() compared to set() without specifying a document ID?

A Both add() and set() automatically generate unique document IDs if none is specified.

B add() automatically generates a unique document ID; set() requires you to specify the document ID explicitly.

C add() requires you to specify the document ID; set() automatically generates a unique document ID.

D Neither add() nor set() can create documents without specifying a document ID.

💡 Hint

Think about which method is designed for quick document creation without manual ID management.

The add() method creates a new document with a unique ID generated by Firestore automatically. The set() method requires you to specify the document ID explicitly when creating a document.

❓ Configuration

intermediate

Using set() to overwrite vs merge document data

When using set() in Firestore, how can you update only specific fields without overwriting the entire document?

A Use set() without any options; it automatically merges fields.

B Use add() instead of set() to update specific fields without overwriting.

C Use set() with the option <code>{ merge: true }</code> to update specific fields without overwriting the whole document.

D Use set() with the option <code>{ overwrite: false }</code> to update specific fields.

💡 Hint

Check the options object passed to set() for controlling overwrite behavior.

By default, set() overwrites the entire document. Passing { merge: true } merges the new data with existing fields, updating only specified fields.

❓ Architecture

advanced

Choosing add() vs set() for user-generated content

You are designing a Firestore database to store user comments. Users can add comments freely. Which method is best to add new comments to the comments collection and why?

A Use add() but manually generate IDs to control comment IDs.

B Use set() with a user-generated ID to keep track of comments by user ID.

C Use set() with a timestamp as the document ID to order comments chronologically.

D Use add() to let Firestore generate unique IDs automatically, ensuring no ID conflicts and easy scalability.

💡 Hint

Consider which method simplifies adding many documents without manual ID management.

Using add() is best for user-generated content like comments because it automatically generates unique IDs, avoiding conflicts and simplifying writes.

❓ security

advanced

Security implications of using set() with client-generated IDs

What is a potential security risk when clients use set() with client-generated document IDs in Firestore?

A Clients might overwrite or delete documents they should not have access to by guessing document IDs.

B Clients cannot write data without server approval, so no risk exists.

C Using client-generated IDs causes Firestore to reject writes automatically.

D There is no difference in security risk between add() and set() with client-generated IDs.

💡 Hint

Think about how document IDs control access and data integrity.

If clients generate document IDs, they might guess IDs of other documents and overwrite or delete data they shouldn't access. This risk is reduced when using add() because IDs are generated server-side.

✅ Best Practice

expert

Best practice for atomic document creation with known ID

You need to create a Firestore document with a specific ID only if it does not already exist. Which approach ensures atomic creation without overwriting existing data?

A Use a transaction to check if the document exists, then create it with set() if it does not.

B Use set() with <code>{ merge: true }</code> to create or update the document safely.

C Use add() with the specific ID passed as a parameter to create the document.

D Use set() without options; it will create the document only if it does not exist.

💡 Hint

Consider how to avoid race conditions when creating documents with known IDs.

Using a transaction allows you to atomically check if a document exists and create it only if it does not, preventing overwriting existing data. set() alone cannot guarantee this atomicity.