Read and write permissions in Firebase - Mini Project: Build & Apply

Firebase Read and Write Permissions

📖 Scenario: You are building a simple Firebase database for a small app. You want to control who can read and write data to keep it safe.

🎯 Goal: Set up Firebase security rules to allow everyone to read data but only authenticated users to write data.

📋 What You'll Learn

Create a Firebase database rules object

Add a rule to allow anyone to read data

Add a rule to allow only authenticated users to write data

Use correct Firebase rules syntax

💡 Why This Matters

🌍 Real World

Firebase security rules protect your app's data from unauthorized access and changes.

💼 Career

Understanding and writing Firebase rules is essential for backend and full-stack developers working with Firebase.

1

Create the base Firebase rules structure

Create a variable called rules that holds an object with a rules key. Inside rules, add a ".read" key set to true to allow everyone to read data.

Firebase

const rules = {
  // Your code here
};

Hint

Start by creating the rules object with a rules key. Set ".read" to true to allow all reads.

2

Add write permission condition

Inside the rules object, add a ".write" key. Set it to auth != null to allow only authenticated users to write data.

Firebase

const rules = {
  rules: {
    ".read": true,
    // Your code here
  }
};

Hint

Use the string "auth != null" to check if the user is signed in before allowing writes.

3

Wrap rules in Firebase JSON format

Make sure the rules variable is a valid JSON object for Firebase security rules. It should have a top-level rules key with ".read" and ".write" keys inside.

Firebase

const rules = {
  rules: {
    ".read": true,
    ".write": "auth != null"
  }
};
// Your code here

Hint

Firebase rules are JSON objects. Check your syntax carefully.

4

Finalize and export the rules

Add a line to export the rules variable as the default export so it can be used in Firebase configuration.

Firebase

const rules = {
  rules: {
    ".read": true,
    ".write": "auth != null"
  }
};
// Your code here

Hint

Use export default rules; to make the rules available for Firebase.

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