Rule syntax and structure in Firebase - Commands & Configuration

Introduction

Firebase security rules control who can read or write data in your database. They use a simple structure to define permissions based on conditions.

When you want to protect your Firebase database from unauthorized access

When you need to allow users to read or write only their own data

When you want to enforce data validation before saving it

When you want to restrict access based on user authentication status

When you want to control access to specific parts of your database

Config File - firestore.rules

firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
    match /public/{document=**} {
      allow read: if true;
      allow write: if false;
    }
  }
}

rules_version: Specifies the version of the rules syntax.

service cloud.firestore: Defines rules for Firestore database.

match /databases/{database}/documents: Applies rules to all documents in the database.

match /users/{userId}: Matches documents in the users collection, allowing read and write only if the user is authenticated and owns the data.

match /public/{document=**}: Matches all documents in the public collection, allowing anyone to read but no one to write.

Commands

Deploys the Firestore security rules to your Firebase project to enforce access control.

Terminal

firebase deploy --only firestore:rules

Expected Output

=== Deploying to 'your-project-id'...



i  deploying firestore



✔  firestore: rules deployed successfully



✔  Deploy complete!

→

--only firestore:rules - Deploy only Firestore security rules without affecting other Firebase services

Starts the local Firestore emulator to test your security rules without affecting the live database.

Terminal

firebase emulators:start --only firestore

Expected Output

i  emulators: Starting emulators: firestore

✔  firestore emulator running at http://localhost:8080



All emulators started, it is now safe to connect.

→

--only firestore - Start only the Firestore emulator

Runs tests against your Firestore rules using a test file to verify permissions work as expected.

Terminal

firebase firestore:rules:test --rules=firestore.rules --file=tests.json

Expected Output

Running Firestore rules tests...

All tests passed successfully.

Key Concept

If you remember nothing else from this pattern, remember: Firebase rules use simple match blocks and conditions to control who can read or write your data.

Common Mistakes

Writing rules without checking if the user is authenticated

This allows anyone to access or modify your data, causing security risks.

Always include a condition like 'request.auth != null' to ensure only logged-in users can access data.

Using incorrect path variables in match statements

Rules won't apply correctly, leading to unexpected access or denials.

Use curly braces {} to capture variables and reference them properly in conditions.

Not deploying updated rules after changes

Your changes won't take effect, leaving old rules active.

Run 'firebase deploy --only firestore:rules' every time you update your rules.

Summary

Write Firebase rules using match blocks to specify which documents they apply to.

Use conditions inside allow statements to control read and write access.

Deploy rules with 'firebase deploy --only firestore:rules' to enforce them.

Practice

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

easy

A. To create new collections automatically

B. To define user roles and permissions

C. To write data to the database

D. To specify which data paths the rules apply to

Solution

1. Step 1: Understand the role of match blocks

   The match block is used to specify the path in the database where the rules apply.

2. Step 2: Differentiate from other rule parts

   Other parts like allow control access, but match targets the data location.

3. Final Answer:

   To specify which data paths the rules apply to -> Option D

4. Quick Check:

   match = data path selector [OK]

Hint: Remember: match points to data location [OK]

Common Mistakes:

• Confusing match with allow
• Thinking match defines user roles
• Assuming match writes data

2. Which of the following is the correct syntax to allow read access to all users in a Firebase rule?

easy

A. allow read { true }

B. allow read: if true;

C. allow read = true;

D. allow read if (true)

Solution

1. Step 1: Recall correct allow syntax

   The correct syntax uses a colon and condition: allow read: if true;

2. Step 2: Check other options for syntax errors

   Options A, B, and D use incorrect symbols or missing keywords.

3. Final Answer:

   allow read: if true; -> Option B

4. Quick Check:

   Correct allow syntax uses colon and if [OK]

Hint: Use colon and 'if' for conditions in allow [OK]

Common Mistakes:

• Using '=' instead of ':'
• Omitting 'if' keyword
• Using braces instead of semicolon

3. Given the rule snippet:

match /posts/{postId} {
  allow read: if request.auth != null;
  allow write: if request.auth.uid == resource.data.authorId;
}

What happens if a logged-in user tries to write to a post they did NOT create?

medium

A. Write is denied because user is not the author

B. Write is allowed because user is logged in

C. Write is allowed only if postId matches user ID

D. Write is denied because read access is restricted

Solution

1. Step 1: Analyze write condition

   Write is allowed only if request.auth.uid == resource.data.authorId, meaning user must be the author.

2. Step 2: Apply condition to user not author

   If user is logged in but not the author, condition fails, so write is denied.

3. Final Answer:

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

4. Quick Check:

   Write allowed only for author = A [OK]

Hint: Check if user ID matches author ID for write [OK]

Common Mistakes:

• Assuming any logged-in user can write
• Confusing read and write conditions
• Ignoring resource data in condition

4. Identify the error in this Firebase rule snippet:

match /users/{userId} {
  allow read if request.auth.uid == userId;
  allow write: if request.auth.uid == userId;
}

medium

A. Missing colon after 'allow read'

B. Incorrect variable name 'userId'

C. Missing semicolon after 'allow write' condition

D. Using '==' instead of '=' in conditions

Solution

1. Step 1: Check syntax for 'allow' statements

   Each allow must have a colon before the condition, e.g., allow read: if ...;

2. Step 2: Identify missing colon

   The first line misses colon after allow read, causing syntax error.

3. Final Answer:

   Missing colon after 'allow read' -> Option A

4. Quick Check:

   Colon required after allow keyword [OK]

Hint: Always put colon after allow before condition [OK]

Common Mistakes:

• Omitting colon after allow
• Confusing '=' and '=='
• Forgetting semicolon at line end

5. You want to write a rule that allows users to read their own profile data but only write if their profile contains a non-empty 'name' field. Which rule snippet correctly enforces this?

hard

A. match /profiles/{userId} {
allow read: if request.auth.uid == userId;
allow write: if request.resource.data.name != "";
}

B. match /profiles/{userId} {
allow read: if request.auth.uid == userId;
allow write: if request.auth.uid == userId || request.resource.data.name != "";
}

C. match /profiles/{userId} {
allow read: if request.auth.uid == userId;
allow write: if request.auth.uid == userId && request.resource.data.name != "";
}

D. match /profiles/{userId} {
allow read: if true;
allow write: if request.auth.uid == userId && request.resource.data.name == "";
}

Solution

1. Step 1: Understand read condition

   Users can read only their own profile: request.auth.uid == userId.

2. Step 2: Understand write condition

   Write allowed only if user owns profile AND 'name' field is not empty: request.auth.uid == userId && request.resource.data.name != "".

3. Step 3: Check options for correct logic

   match /profiles/{userId} { allow read: if request.auth.uid == userId; allow write: if request.auth.uid == userId && request.resource.data.name != ""; } matches both conditions correctly; others use wrong logic or allow too much.

4. Final Answer:

   match /profiles/{userId} { allow read: if request.auth.uid == userId; allow write: if request.auth.uid == userId && request.resource.data.name != ""; } -> Option C

5. Quick Check:

   Use AND for ownership and non-empty name [OK]

Hint: Use AND (&&) to combine ownership and data checks [OK]

Common Mistakes:

• Using OR (||) instead of AND (&&)
• Allowing write without ownership check
• Checking for empty string incorrectly