Common rule patterns in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of Firebase security rules?

Firebase security rules control who can read or write data in your database or storage. They protect your data from unauthorized access.

beginner

Explain the 'allow read, write: if false;' rule pattern.

This rule denies all reads and writes. It is used to block access completely until you define more specific rules.

beginner

What does 'allow read, write: if request.auth != null;' do?

It allows only authenticated users to read and write data. Users must be signed in to access the data.

intermediate

How do you allow users to only modify their own data in Firebase rules?

Use a rule that checks if the user ID matches the data owner ID, for example: 'allow write: if request.auth.uid == resource.data.userId;'.

intermediate

What is the benefit of using data validation in Firebase rules?

Data validation checks that data meets certain conditions before it is saved, helping keep your data clean and consistent.

Which Firebase rule pattern blocks all access to data?

A allow read, write: if true;

B allow read, write: if false;

C allow read: if request.auth != null;

D allow write: if request.auth.uid == resource.data.userId;

The rule 'allow read, write: if false;' denies all reads and writes, blocking all access.

What does 'request.auth != null' check in Firebase rules?

A If the user is an admin

B If the data exists

C If the request is from the server

D If the user is authenticated

It checks if the user is signed in (authenticated).

How can you restrict write access to only the owner of the data?

A allow write: if request.auth.uid == resource.data.userId;

B allow write: if true;

C allow write: if request.time < timestamp;

D allow write: if request.auth == null;

This rule allows writes only if the signed-in user's ID matches the data owner's ID.

What is the role of data validation in Firebase rules?

A To allow all reads

B To authenticate users

C To check data format before saving

D To deny all writes

Data validation ensures data meets conditions before it is saved.

Which rule allows anyone to read data but only authenticated users to write?

A allow read: if true; allow write: if request.auth != null;

B allow read, write: if request.auth != null;

C allow read, write: if false;

D allow write: if true;

This pattern lets anyone read but restricts writes to signed-in users.

Describe how Firebase rules can restrict data access to only authenticated users.

Think about how to check if a user is signed in.

Explain how to write a Firebase rule that lets users only change their own data.

Consider comparing user IDs in the rule.

Practice

1. What is the main purpose of Firebase security rules in a database?

easy

A. To control who can read or write data

B. To speed up database queries

C. To store user passwords securely

D. To backup data automatically

Solution

1. Step 1: Understand Firebase security rules role

   Firebase security rules define who can access or change data in your database.

2. Step 2: Identify the main function

   The main function is to control read and write access to protect data.

3. Final Answer:

   To control who can read or write data -> Option A

4. Quick Check:

   Security rules = access control [OK]

Hint: Rules control data access permissions quickly [OK]

Common Mistakes:

• Confusing rules with data backup
• Thinking rules speed up queries
• Assuming rules store passwords

2. Which of the following is the correct syntax to allow only authenticated users to read data in Firebase rules?

easy

A. allow read: if request.auth != null;

B. allow read: if user.loggedIn == true;

C. allow read: if auth.user == true;

D. allow read: if request.user != null;

Solution

1. Step 1: Identify correct Firebase auth syntax

   Firebase uses request.auth to check if a user is authenticated.

2. Step 2: Match syntax to allow read only if authenticated

   The correct condition is request.auth != null meaning user is logged in.

3. Final Answer:

   allow read: if request.auth != null; -> Option A

4. Quick Check:

   request.auth != null means user logged in [OK]

Hint: Use request.auth to check user authentication [OK]

Common Mistakes:

• Using incorrect variable names like user.loggedIn
• Checking auth.user instead of request.auth
• Confusing request.user with request.auth

3. Given this Firebase rule snippet:

allow write: if request.auth.uid == resource.data.ownerId;

What does this rule enforce?

medium

A. No one can write to the data

B. Only the owner of the data can write to it

C. Anyone can write if authenticated

D. Only admins can write to the data

Solution

1. Step 1: Understand the condition in the rule

   The rule checks if the user's ID matches the ownerId stored in the data.

2. Step 2: Interpret the effect of the condition

   This means only the user who owns the data (ownerId) can write to it.

3. Final Answer:

   Only the owner of the data can write to it -> Option B

4. Quick Check:

   request.auth.uid == resource.data.ownerId means owner only [OK]

Hint: Match request.auth.uid with data ownerId to allow write [OK]

Common Mistakes:

• Assuming all authenticated users can write
• Thinking admins are required
• Believing no one can write

4. Identify the error in this Firebase rule snippet:

allow read: if request.auth.uid = resource.data.ownerId;

medium

A. request.auth.uid is not a valid property

B. Missing semicolon at the end of the line

C. Using single equals (=) instead of double equals (==) for comparison

D. resource.data.ownerId cannot be used in rules

Solution

1. Step 1: Check the comparison operator used

   The rule uses a single equals sign (=) which is assignment, not comparison.

2. Step 2: Correct operator for comparison in Firebase rules

   Firebase rules require double equals (==) to compare values.

3. Final Answer:

   Using single equals (=) instead of double equals (==) for comparison -> Option C

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == to compare values in rules, not = [OK]

Common Mistakes:

• Confusing assignment (=) with comparison (==)
• Thinking semicolon is mandatory
• Believing request.auth.uid is invalid

5. You want to allow users to update their own profile but prevent them from changing their user ID. Which Firebase rule pattern correctly enforces this?

hard

A. allow update: if request.auth.uid == request.resource.data.userId;

B. allow update: if request.resource.data.userId == null;

C. allow update: if request.auth.uid != resource.data.userId;

D. allow update: if request.auth.uid == resource.data.userId && request.resource.data.userId == resource.data.userId;

Solution

1. Step 1: Ensure user can update only their own profile

   The rule must check that the authenticated user matches the existing userId in data.

2. Step 2: Prevent changing the userId field

   Check that the new data's userId matches the old data's userId to prevent changes.

3. Final Answer:

   allow update: if request.auth.uid == resource.data.userId && request.resource.data.userId == resource.data.userId; -> Option D

4. Quick Check:

   Match auth.uid and keep userId unchanged [OK]

Hint: Check auth.uid matches and userId stays same [OK]

Common Mistakes:

• Allowing userId to be changed
• Not verifying user owns the profile
• Using != instead of == for ownership check