Why RLS is essential for multi-user apps in Supabase - See It in Action

Why RLS is essential for multi-user apps

📖 Scenario: You are building a multi-user task management app using Supabase. Each user should only see and manage their own tasks. To keep data safe and private, you need to control who can access which data.

🎯 Goal: Learn how to set up Row Level Security (RLS) in Supabase to restrict data access so that each user can only see their own tasks.

📋 What You'll Learn

Create a tasks table with columns id, user_id, and task

Enable Row Level Security (RLS) on the tasks table

Create a policy that allows users to select only their own tasks

Test the policy by simulating a user query

💡 Why This Matters

🌍 Real World

Multi-user apps like task managers, social networks, or any app where users must only access their own data need RLS to keep data private and secure.

💼 Career

Understanding RLS is essential for backend developers and cloud engineers working with databases and user data security in modern cloud apps.

1

Create the tasks table

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

Supabase

-- Your SQL code here to create the tasks table

Hint

Use CREATE TABLE with the specified columns and types.

2

Enable Row Level Security (RLS)

Write SQL to enable Row Level Security on the tasks table using ALTER TABLE tasks ENABLE ROW LEVEL SECURITY;.

Supabase

CREATE TABLE tasks (
  id SERIAL PRIMARY KEY,
  user_id UUID NOT NULL,
  task TEXT NOT NULL
);
-- Enable RLS on tasks table below

Hint

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

3

Create a policy to allow users to select their own tasks

Write SQL to create a policy named select_own_tasks on tasks that allows SELECT only when user_id = auth.uid().

Supabase

CREATE TABLE tasks (
  id SERIAL PRIMARY KEY,
  user_id UUID NOT NULL,
  task TEXT NOT NULL
);

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

Hint

Use CREATE POLICY with FOR SELECT and USING (user_id = auth.uid()).

4

Test the RLS policy with a user query

Write SQL to select all tasks from tasks where the current user can only see their own tasks. Use SELECT * FROM tasks; assuming RLS is active.

Supabase

CREATE TABLE tasks (
  id SERIAL PRIMARY KEY,
  user_id UUID NOT NULL,
  task TEXT NOT NULL
);

ALTER TABLE tasks ENABLE ROW LEVEL SECURITY;

CREATE POLICY select_own_tasks ON tasks
  FOR SELECT
  USING (user_id = auth.uid());

-- Write a SELECT query to test RLS below

Hint

Use a simple SELECT * FROM tasks; query to test RLS.

Practice

1. Why is Row Level Security (RLS) important in multi-user apps using Supabase?

easy

A. It ensures each user can only access their own data.

B. It speeds up database queries for all users.

C. It automatically backs up user data regularly.

D. It allows users to share data without restrictions.

Solution

1. Step 1: Understand RLS purpose

   RLS restricts data access so users see only their own rows in a shared table.

2. Step 2: Connect to multi-user app needs

   In apps with many users, RLS prevents users from seeing others' private data.

3. Final Answer:

   It ensures each user can only access their own data. -> Option A

4. Quick Check:

   RLS = User data privacy [OK]

Hint: RLS = data access control per user [OK]

Common Mistakes:

• Thinking RLS improves speed
• Believing RLS backs up data
• Assuming RLS allows open data sharing

2. Which of the following is the correct way to enable RLS on a Supabase table named profiles?

easy

A. ALTER TABLE profiles ENABLE ROW LEVEL SECURITY;

B. CREATE RLS ON profiles;

C. ENABLE RLS profiles;

D. SET RLS ON profiles;

Solution

1. Step 1: Recall SQL syntax for enabling RLS

   The correct command is ALTER TABLE <table> ENABLE ROW LEVEL SECURITY;.

2. Step 2: Match syntax to options

   ALTER TABLE profiles ENABLE ROW LEVEL SECURITY; matches the exact syntax to enable RLS on the profiles table.

3. Final Answer:

   ALTER TABLE profiles ENABLE ROW LEVEL SECURITY; -> Option A

4. Quick Check:

   Enable RLS = ALTER TABLE ... ENABLE ROW LEVEL SECURITY [OK]

Hint: Use ALTER TABLE to enable RLS [OK]

Common Mistakes:

• Using CREATE instead of ALTER
• Missing TABLE keyword
• Wrong command order or keywords

3. Given this RLS policy on a tasks table:

CREATE POLICY "Users can view their tasks" ON tasks FOR SELECT USING (user_id = auth.uid());

What will happen if user with ID '123' queries SELECT * FROM tasks;?

medium

A. They will see all tasks regardless of user_id.

B. They will see only tasks where user_id is '123'.

C. They will get an error about missing permissions.

D. They will see no tasks at all.

Solution

1. Step 1: Understand the policy condition

   The policy allows SELECT only if user_id = auth.uid(), meaning user sees rows matching their own ID.

2. Step 2: Apply to user ID '123'

   User '123' will see only rows where user_id equals '123'.

3. Final Answer:

   They will see only tasks where user_id is '123'. -> Option B

4. Quick Check:

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

Hint: RLS filters rows by matching user ID [OK]

Common Mistakes:

• Assuming all rows are visible
• Expecting permission error without policy
• Thinking no rows will show

4. You created this RLS policy:

CREATE POLICY "Allow users to update their profiles" ON profiles FOR UPDATE USING (user_id = auth.uid());

But users report they cannot update their profiles. What is the most likely cause?

medium

A. The auth.uid() function is misspelled.

B. The policy uses FOR UPDATE instead of FOR SELECT.

C. The user_id column does not exist in profiles.

D. RLS is not enabled on the profiles table.

Solution

1. Step 1: Check RLS enablement

   Even with a policy, RLS must be enabled on the table to enforce it.

2. Step 2: Identify cause of update failure

   If RLS is disabled, policies don't apply, so updates may be blocked or unrestricted incorrectly.

3. Final Answer:

   RLS is not enabled on the profiles table. -> Option D

4. Quick Check:

   Enable RLS before policies work [OK]

Hint: Enable RLS before creating policies [OK]

Common Mistakes:

• Confusing policy action with table RLS status
• Assuming policy syntax error causes issue
• Ignoring RLS enablement step

5. You want to allow users to see their own orders and also let admins see all orders. Which RLS policy setup achieves this safely?

hard

A. CREATE POLICY "Admins only" ON orders FOR SELECT USING (auth.role() = 'admin');

B. CREATE POLICY "Users only" ON orders FOR SELECT USING (user_id = auth.uid());

C. CREATE POLICY "Users and admins" ON orders FOR SELECT USING (user_id = auth.uid() OR auth.role() = 'admin');

D. No RLS needed if admins have full access.

Solution

1. Step 1: Identify user and admin access needs

   Users see their own orders; admins see all orders.

2. Step 2: Write policy condition combining both

   Use user_id = auth.uid() OR auth.role() = 'admin' to allow both cases.

3. Step 3: Verify policy covers both roles safely

   CREATE POLICY "Users and admins" ON orders FOR SELECT USING (user_id = auth.uid() OR auth.role() = 'admin'); correctly implements this combined condition.

4. Final Answer:

   CREATE POLICY "Users and admins" ON orders FOR SELECT USING (user_id = auth.uid() OR auth.role() = 'admin'); -> Option C

5. Quick Check:

   Combine user and admin checks in policy [OK]

Hint: Use OR to combine user and admin access in RLS [OK]

Common Mistakes:

• Creating separate policies without OR condition
• Assuming admins need no RLS
• Allowing all users to see all data