Why RLS is essential for multi-user apps in Supabase - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable Row Level Security (RLS) on a Supabase table.

Supabase

alter table messages [1] enable row level security;

Drag options to blanks, or click blank then click option'

Aset

Balter

Ccreate

Denable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' or 'create' instead of 'enable' causes syntax errors.

Forgetting to use 'alter table' before enabling RLS.

To activate RLS on a table, you use the ENABLE keyword with ALTER TABLE.

2fill in blank

medium

Complete the policy to allow users to select only their own rows.

Supabase

create policy "Users can select their own messages" on messages for select using (user_id = [1]);

Drag options to blanks, or click blank then click option'

Aauth.uid()

Bcurrent_user

Csession_user

Duser()

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic SQL functions like current_user which do not return the Supabase user ID.

Leaving the condition empty or incorrect.

The function auth.uid() returns the current user's ID in Supabase, which is used to filter rows.

3fill in blank

hard

Fix the error in the policy condition to correctly restrict updates to the user's own rows.

Supabase

create policy "Users can update their own messages" on messages for update using (user_id [1] auth.uid());

Drag options to blanks, or click blank then click option'

A=

B==

C!=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' causes syntax errors in SQL.

Using '!=' or '<>' would invert the condition, allowing wrong access.

In SQL, the equality operator is a single equals sign =, not double equals.

4fill in blank

hard

Fill both blanks to create a policy that allows inserting messages only if the user_id matches the current user.

Supabase

create policy "Users can insert their own messages" on messages for insert with check (user_id [1] [2]);

Drag options to blanks, or click blank then click option'

A=

Bauth.uid()

Ccurrent_user

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' reverses the logic.

Using 'current_user' instead of 'auth.uid()' does not work in Supabase.

The policy checks that user_id = auth.uid() to allow inserts only by the owner.

5fill in blank

hard

Fill all three blanks to create a policy that allows deleting messages only if the user owns the message and the message is not older than 7 days.

Supabase

create policy "Users can delete recent own messages" on messages for delete using (user_id [1] [2] and created_at > now() - interval '[3] days');

Drag options to blanks, or click blank then click option'

A=

Bauth.uid()

C7

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' reverses ownership check.

Using a wrong number of days or omitting the interval causes logic errors.

This policy ensures the user can delete only their own messages (user_id = auth.uid()) and only if the message is newer than 7 days.

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