Using auth.uid() in policies in Supabase

Introduction

We use auth.uid() in policies to check who is making a request. This helps keep data safe by allowing only the right users to access or change it.

When you want users to see only their own data in a database.

When you want to let users update their profile but not others'.

When you want to restrict deleting records to the owner only.

When you want to log actions and link them to the user who did them.

Syntax

Supabase

using (auth.uid() = user_id)

auth.uid() returns the unique ID of the logged-in user making the request.

You compare auth.uid() to a column in your table that stores the owner's user ID.

Examples

This policy allows access only if the logged-in user's ID matches the user_id in the profile table.

Supabase

using (auth.uid() = profile.user_id)

This lets a user access only their own orders by matching their ID to customer_id in the orders table.

Supabase

using (auth.uid() = orders.customer_id)

Sample Program

This policy allows users to read only their own profile data by checking if their logged-in ID matches the user_id column.

Supabase

create policy "Users can select their own profiles" on profiles
  for select
  using (auth.uid() = user_id);

Important Notes

Make sure your table has a column that stores the user ID to compare with auth.uid().

If auth.uid() is null, the user is not logged in, so the policy denies access.

Test your policies carefully to avoid accidentally exposing data.

Summary

auth.uid() gives the current user's ID.

Use it in policies to allow users to access only their own data.

Always compare auth.uid() to a user ID column in your table.

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