Read and write permissions in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are read permissions in Firebase?

Read permissions control who can view or access data stored in Firebase. They decide if a user or app can fetch data.

beginner

What do write permissions allow in Firebase?

Write permissions allow users or apps to add, change, or delete data in Firebase. They control who can update the database.

intermediate

How do Firebase rules use read and write permissions?

Firebase rules use 'read' and 'write' keywords to set conditions. These conditions decide if a user can read or write data at a specific location.

intermediate

Example: What does this Firebase rule mean? {"rules": {"messages": {".read": "auth != null", ".write": "auth != null"}}}

This rule means only authenticated users (users who signed in) can read or write data under 'messages'.

beginner

Why is it important to set proper read and write permissions in Firebase?

Proper permissions protect your data from unauthorized access or changes. They keep your app safe and your users' data private.

In Firebase, what does the '.read' rule control?

A Who can delete data

B Who can create new users

C Who can view data

D Who can update Firebase rules

The '.read' rule controls who can view or fetch data from Firebase.

What does setting '.write' to 'auth != null' mean in Firebase rules?

A Only authenticated users can write data

B Only admins can write data

C No one can write data

D Anyone can write data

The condition 'auth != null' means the user must be signed in to write data.

If a Firebase rule has '.read': false, what happens?

A No one can read data

B Only admins can read data

C Everyone can read data

D Only authenticated users can read data

Setting '.read' to false blocks all users from reading data.

Why should you avoid setting '.write' to true for all users?

A It disables authentication

B It makes the app slower

C It prevents users from reading data

D It can cause data loss or unauthorized changes

Allowing everyone to write can lead to data loss or security risks.

Which Firebase feature helps you test read and write permissions before deploying?

A Firebase Hosting

B Firebase Emulator Suite

C Firebase Analytics

D Firebase Cloud Messaging

Firebase Emulator Suite lets you safely test your rules locally.

Explain how read and write permissions work in Firebase and why they are important.

Think about who can see and change your app's data.

Describe a simple Firebase rule that allows only signed-in users to read and write data.

Focus on the condition that checks if a user is signed in.

Practice

1. What is the main purpose of Firebase read and write permissions?

easy

A. To control who can view or change data in the database

B. To speed up data retrieval from the database

C. To backup data automatically

D. To create new database tables

Solution

1. Step 1: Understand permission roles

   Read and write permissions define who can access or modify data.

2. Step 2: Identify the purpose

   They protect data by controlling access, not by speeding up or backing up data.

3. Final Answer:

   To control who can view or change data in the database -> Option A

4. Quick Check:

   Permissions control access = A [OK]

Hint: Permissions control access, not speed or backups [OK]

Common Mistakes:

• Confusing permissions with performance features
• Thinking permissions create database structure
• Assuming permissions handle backups

2. Which Firebase rule syntax correctly allows only authenticated users to read data?

easy

A. "read": "true"

B. "read": "auth != null"

C. "read": "auth == null"

D. "read": "false"

Solution

1. Step 1: Understand auth variable

   In Firebase rules, auth != null means user is logged in.

2. Step 2: Match condition to requirement

   Allowing read only if auth != null means only authenticated users can read.

3. Final Answer:

   "read": "auth != null" -> Option B

4. Quick Check:

   auth != null means logged in = C [OK]

Hint: auth != null means user is logged in [OK]

Common Mistakes:

• Using auth == null which allows unauthenticated users
• Setting read to true allows everyone
• Setting read to false blocks all reads

3. Given this Firebase rule snippet:

{
  "rules": {
    "messages": {
      ".read": "auth != null",
      ".write": "auth.uid == data.child('owner').val()"
    }
  }
}

What happens if a logged-in user tries to write a message where they are not the owner?

medium

A. Write is denied because user is not the owner

B. Write is allowed because user is logged in

C. Write is allowed for all users

D. Write causes a syntax error

Solution

1. Step 1: Analyze write condition

   Write is allowed only if auth.uid equals the owner field in data.

2. Step 2: Check user ownership

   If user is not owner, condition fails, so write is denied.

3. Final Answer:

   Write is denied because user is not the owner -> Option A

4. Quick Check:

   Write allowed only if user owns data = D [OK]

Hint: Write allowed only if auth.uid matches owner [OK]

Common Mistakes:

• Assuming logged-in users can write anywhere
• Confusing read and write rules
• Ignoring the owner check in write condition

4. Identify the error in this Firebase rule snippet:

{
  "rules": {
    "posts": {
      ".read": "auth != null",
      ".write": "auth.uid = data.child('author').val()"
    }
  }
}

medium

A. data.child() cannot be used in rules

B. Missing quotes around auth.uid

C. auth != null is invalid syntax

D. Using single equals (=) instead of double equals (==) in write condition

Solution

1. Step 1: Check write condition syntax

   Single equals (=) is assignment, not comparison in Firebase rules.

2. Step 2: Correct comparison operator

   Use double equals (==) to compare auth.uid and data.child('author').val().

3. Final Answer:

   Using single equals (=) instead of double equals (==) in write condition -> Option D

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == for comparison, not = [OK]

Common Mistakes:

• Confusing assignment (=) with comparison (==)
• Thinking auth.uid needs quotes
• Believing data.child() is invalid in rules

5. You want to allow users to read all their own profile data but prevent others from reading it. Which Firebase rule correctly enforces this?

{
  "rules": {
    "profiles": {
      "$userId": {
        ".read": "?",
        ".write": "auth.uid == $userId"
      }
    }
  }
}

hard

A. "auth != null"

B. "true"

C. "auth.uid == $userId"

D. "auth.uid != $userId"

Solution

1. Step 1: Understand read condition need

   Users should read only their own profile, so read must check auth.uid equals $userId.

2. Step 2: Match read condition to requirement

   Setting ".read": "auth.uid == $userId" restricts read access to owner only.

3. Final Answer:

   "auth.uid == $userId" -> Option C

4. Quick Check:

   Read allowed only if user matches profile ID = B [OK]

Hint: Read only if auth.uid equals profile ID [OK]

Common Mistakes:

• Using auth != null allows all logged-in users to read
• Setting read to true allows everyone
• Using auth.uid != $userId allows others to read