Using auth.uid() in policies in Supabase - Mini Project: Build & Apply

Using auth.uid() in Supabase Row Level Security Policies

📖 Scenario: You are building a simple task management app using Supabase. Each user should only see and modify their own tasks. To enforce this, you will use Supabase's Row Level Security (RLS) policies with auth.uid() to restrict access.

🎯 Goal: Create a Supabase table called tasks with columns for id, user_id, and task_description. Then write a Row Level Security policy that allows users to select only their own tasks by comparing user_id with auth.uid().

📋 What You'll Learn

Create a table named tasks with columns id (UUID primary key), user_id (UUID), and task_description (text).

Enable Row Level Security (RLS) on the tasks table.

Create a policy named Users can select their own tasks that allows SELECT only when user_id = auth.uid().

Use exact column and table names as specified.

💡 Why This Matters

🌍 Real World

This project shows how to secure user data in multi-user applications by using Supabase's built-in authentication and Row Level Security policies.

💼 Career

Understanding how to use auth.uid() in policies is essential for backend developers and cloud engineers working with Supabase or similar backend-as-a-service platforms to enforce data privacy and security.

1

Create the tasks table

Write a SQL statement to create a table called tasks with columns: id as UUID primary key, user_id as UUID, and task_description as text.

Supabase

-- Your SQL code here to create the tasks table

Hint

Use CREATE TABLE tasks and define the columns with their types exactly as specified.

2

Enable Row Level Security on tasks

Write a SQL statement to enable Row Level Security (RLS) on the tasks table.

Supabase

CREATE TABLE tasks (
    id UUID PRIMARY KEY,
    user_id UUID,
    task_description TEXT
);
-- Enable RLS on tasks table below

Hint

Use ALTER TABLE tasks ENABLE ROW LEVEL SECURITY; to turn on RLS.

3

Create a SELECT policy using auth.uid()

Write a SQL statement to create a policy named Users can select their own tasks on the tasks table that allows SELECT only when user_id = auth.uid().

Supabase

CREATE TABLE tasks (
    id UUID PRIMARY KEY,
    user_id UUID,
    task_description TEXT
);

ALTER TABLE tasks ENABLE ROW LEVEL SECURITY;
-- Create SELECT policy below

Hint

Use CREATE POLICY with the exact name and condition user_id = auth.uid().

4

Allow users to insert their own tasks

Write a SQL statement to create a policy named Users can insert their own tasks on the tasks table that allows INSERT only when user_id = auth.uid().

Supabase

CREATE TABLE tasks (
    id UUID PRIMARY KEY,
    user_id UUID,
    task_description TEXT
);

ALTER TABLE tasks ENABLE ROW LEVEL SECURITY;

CREATE POLICY "Users can select their own tasks" ON tasks
FOR SELECT USING (user_id = auth.uid());
-- Create INSERT policy below

Hint

Use CREATE POLICY with the exact name and condition WITH CHECK (user_id = auth.uid()) for INSERT.

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