Using auth.uid() in policies in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does auth.uid() represent in Supabase policies?

auth.uid() returns the unique user ID of the currently authenticated user making the request. It helps identify who is accessing the data.

beginner

How is auth.uid() commonly used in row-level security policies?

It is used to restrict access so users can only see or modify rows that belong to their own user ID, ensuring data privacy.

beginner

Example: What does this policy condition mean? user_id = auth.uid()

This means the row can only be accessed if the user_id column matches the current user's ID, allowing users to access only their own data.

intermediate

Can auth.uid() be used in both SELECT and UPDATE policies?

Yes, auth.uid() can be used in any policy type (SELECT, INSERT, UPDATE, DELETE) to control access based on the authenticated user.

intermediate

Why is using auth.uid() in policies considered a best practice?

Because it enforces security by ensuring users only access their own data, preventing unauthorized access and protecting user privacy.

What does auth.uid() return in a Supabase policy?

A The unique ID of the authenticated user

B The user's email address

C The current timestamp

D The database table name

auth.uid() returns the unique user ID of the authenticated user making the request.

Which policy condition restricts access to rows owned by the current user?

A current_timestamp > created_at

B auth.email() = user_email

C user_id = auth.uid()

D user_role = 'admin'

The condition user_id = auth.uid() ensures users access only their own rows.

Can auth.uid() be used in an UPDATE policy to restrict who can modify data?

A No

B Only in DELETE policies

C Only in SELECT policies

D Yes

auth.uid() can be used in any policy type including UPDATE to control access.

Why is it important to use auth.uid() in policies?

A To ensure users only access their own data

B To speed up database queries

C To log user activity

D To encrypt data

Using auth.uid() enforces data privacy by limiting access to the user's own data.

If a policy uses auth.uid(), what happens if the user is not authenticated?

A The user is automatically authenticated

B Access is denied

C The policy ignores <code>auth.uid()</code>

D Access is granted to all data

If the user is not authenticated, auth.uid() is null and access is denied by the policy.

Explain how auth.uid() helps secure data access in Supabase policies.

Think about how you keep your personal files private.

Describe a simple policy using auth.uid() to allow users to update only their own records.

Imagine a rule that only lets you edit your own profile.

Practice

1.

What does auth.uid() represent in Supabase row-level security policies?

easy

A. The current logged-in user's unique ID

B. The database server's ID

C. The table's primary key

D. The user's email address

Solution

1. Step 1: Understand the purpose of auth.uid()

   auth.uid() is a function that returns the unique identifier of the currently logged-in user in Supabase.

2. Step 2: Identify what auth.uid() is not

   It does not return server or table IDs, nor user email addresses directly.

3. Final Answer:

   The current logged-in user's unique ID -> Option A

4. Quick Check:

   auth.uid() = current user ID [OK]

Hint: auth.uid() always means the logged-in user's ID [OK]

Common Mistakes:

• Thinking auth.uid() returns email
• Confusing auth.uid() with table keys
• Assuming auth.uid() is server ID

2.

Which of the following is the correct syntax to allow a user to select only their own rows in a table with a user_id column?

CREATE POLICY "Select own rows" ON my_table FOR SELECT USING ( ... );

easy

A. user_id = current_user()

B. auth.uid() = user_email

C. user_id == auth.uid()

D. user_id = auth.uid()

Solution

1. Step 1: Check correct equality operator in policies

   Supabase policies use single equals = for comparison, not double equals ==.

2. Step 2: Match user_id with auth.uid()

   The correct way is to compare the table's user_id column to auth.uid().

3. Final Answer:

   user_id = auth.uid() -> Option D

4. Quick Check:

   Use single = and compare user_id to auth.uid() [OK]

Hint: Use single = to compare user_id with auth.uid() [OK]

Common Mistakes:

• Using double equals == in policy
• Comparing auth.uid() to email column
• Using current_user() instead of auth.uid()

3.

Given this policy on orders table:

CREATE POLICY "Users can select own orders" ON orders FOR SELECT USING (user_id = auth.uid());

What happens if a user with auth.uid() = 'abc123' runs a query to select orders?

medium

A. They see only orders where user_id is 'abc123'

B. They see all orders regardless of user_id

C. They get a syntax error

D. They see no orders at all

Solution

1. Step 1: Understand the policy condition

   The policy allows selecting rows where user_id equals the current user's ID.

2. Step 2: Apply the policy to the user's query

   The user with ID 'abc123' will only see rows where user_id is 'abc123'.

3. Final Answer:

   They see only orders where user_id is 'abc123' -> Option A

4. Quick Check:

   Policy filters rows by user_id = auth.uid() [OK]

Hint: Policy filters rows to match auth.uid() user_id [OK]

Common Mistakes:

• Thinking user sees all rows
• Expecting syntax error from correct policy
• Assuming no rows are returned

4.

Identify the error in this policy that aims to restrict updates to a user's own rows:

CREATE POLICY "Update own rows" ON profiles FOR UPDATE USING (user_id = auth.uid);

medium

A. user_id should be user_email

B. Wrong operator, should use == instead of =

C. Missing parentheses after auth.uid

D. Policy name cannot have spaces

Solution

1. Step 1: Check the usage of auth.uid()

   auth.uid is a function and must be called with parentheses: auth.uid().

2. Step 2: Verify other parts of the policy

   The operator = is correct, user_id is the right column, and policy names can have spaces.

3. Final Answer:

   Missing parentheses after auth.uid -> Option C

4. Quick Check:

   auth.uid() needs parentheses to call function [OK]

Hint: Always call auth.uid() with parentheses [OK]

Common Mistakes:

• Forgetting () after auth.uid
• Using == instead of = in policies
• Changing user_id to email incorrectly

5.

You want to create a policy that allows users to update their own profile only if their email is verified. The profiles table has columns user_id and email_verified (boolean). Which policy condition correctly enforces this?

hard

A. email_verified = true

B. user_id = auth.uid() AND email_verified = true

C. auth.uid() = user_id AND email_verified = false

D. user_id = auth.uid() OR email_verified = true

Solution

1. Step 1: Combine user ownership and email verification

   The policy must check both that the row belongs to the user and that their email is verified.

2. Step 2: Use AND to require both conditions

   Using AND ensures both user_id = auth.uid() and email_verified = true must be true.

3. Final Answer:

   user_id = auth.uid() AND email_verified = true -> Option B

4. Quick Check:

   Use AND to combine user_id and email_verified checks [OK]

Hint: Use AND to require both user_id and email_verified true [OK]

Common Mistakes:

• Using OR instead of AND
• Checking email_verified = false
• Omitting user_id check