Rule syntax and structure in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic structure of a Firebase security rule?

A Firebase security rule has a service block, inside which there are match blocks defining paths, and inside those, allow statements that specify read or write permissions.

beginner

What does the match keyword do in Firebase rules?

The match keyword defines a path in your database or storage where rules apply. It scopes the rules to specific data locations.

beginner

Explain the purpose of allow read: if condition; in Firebase rules.

This statement allows read access to data only if the specified condition evaluates to true. It controls who can read data.

intermediate

How do you specify multiple permissions in one rule?

You can list multiple permissions separated by commas, like allow read, write: if condition;, to apply the same condition to both read and write.

intermediate

What is the role of variables in Firebase rule paths?

Variables in paths, like <code>{userId}</code>, act as placeholders that match any value. They let you write flexible rules for dynamic data locations.

What keyword defines the data path in Firebase security rules?

A path

B allow

C service

D match

The match keyword defines the path where rules apply.

Which statement correctly allows both read and write if a condition is true?

A allow read, write: if condition;

B allow read write if condition;

C allow read or write: if condition;

D allow read if condition; allow write if condition;

You list multiple permissions separated by commas in one allow statement.

What does {userId} represent in a Firebase rule path?

A A variable placeholder

B A fixed folder name

C A function call

D A permission type

Curly braces define variables that match any value in that path segment.

Where do you write Firebase security rules for Firestore?

A Inside <code>service cloud.functions</code> block

B Inside <code>service cloud.firestore</code> block

C Inside <code>service firebase.storage</code> block

D Inside <code>service cloud.database</code> block

Firestore rules go inside the service cloud.firestore block.

What happens if no rule matches a request in Firebase security rules?

A The request is queued

B Access is allowed by default

C Access is denied by default

D The request is logged but allowed

Firebase denies access by default if no rule matches.

Describe the main parts of a Firebase security rule and their roles.

Think about how you specify where and when access is allowed.

Explain how variables in rule paths help manage permissions dynamically.

Imagine you want to write one rule for many users.

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