Update and delete policies in Supabase - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Supabase Update Policies

In Supabase, you want to allow users to update only their own profile data. Which policy condition correctly enforces this?

A auth.role() = 'admin'

B profile.user_id = 'public'

C auth.uid() = profile.user_id

D auth.email() = profile.email

💡 Hint

Think about how to match the logged-in user with the data owner.

The condition auth.uid() = profile.user_id ensures only the user who owns the profile can update it.

❓ service_behavior

intermediate

Effect of Missing Delete Policy

You have a table with no delete policy defined in Supabase. What happens when a user tries to delete a row?

A The delete operation is denied for all users.

B The delete operation is allowed for all users.

C The delete operation is allowed only for admins.

D The delete operation causes a runtime error.

💡 Hint

Consider the default behavior when no policy is set.

Without a delete policy, Supabase denies all delete operations by default for security.

❓ Configuration

advanced

Correct Update Policy Syntax

Which of the following Supabase update policies correctly allows updates only if the user is authenticated and owns the row?

A using: (true), check: (auth.uid() = user_id)

B using: (auth.role() = 'authenticated'), check: (user_id = auth.uid())

C using: (auth.uid() = user_id), check: (true)

D using: (auth.uid() = user_id), check: (auth.uid() = user_id)

💡 Hint

Both using and check must enforce ownership.

The using clause filters rows the user can see, and check validates the update. Both must confirm ownership.

❓ security

advanced

Preventing Unauthorized Deletes

You want to prevent users from deleting rows they do not own. Which policy condition should you add to the delete policy?

A using: (auth.uid() = owner_id)

B using: (auth.role() = 'admin')

C using: (true)

D using: (auth.email() = owner_email)

💡 Hint

Match the logged-in user ID with the row owner ID.

The condition auth.uid() = owner_id ensures only the owner can delete their rows.

❓ Architecture

expert

Combining Update and Delete Policies for Multi-Tenant Security

You manage a multi-tenant app where each tenant's data is separated by tenant_id. Which combined policy setup ensures users can update and delete only their tenant's data?

A Update policy: using: (auth.uid() = user_id and tenant_id = auth.jwt().tenant_id), check: (true); Delete policy: using: (tenant_id = auth.jwt().tenant_id)

B Update policy: using: (auth.uid() = user_id), check: (tenant_id = auth.jwt().tenant_id); Delete policy: using: (tenant_id = auth.jwt().tenant_id and auth.uid() = user_id)

C Update policy: using: (true), check: (tenant_id = auth.jwt().tenant_id); Delete policy: using: (auth.uid() = user_id and tenant_id = auth.jwt().tenant_id)

D Update policy: using: (tenant_id = auth.jwt().tenant_id), check: (user_id = auth.uid()); Delete policy: using: (auth.uid() = user_id)

💡 Hint

Both user ownership and tenant matching must be enforced in both policies.

Option B correctly enforces that the user owns the row and belongs to the tenant for both update and delete operations.

Practice

1. What is the main purpose of an UPDATE policy in Supabase?

easy

A. To control who can add new data to a table

B. To control who can read data from a table

C. To control who can change existing data in a table

D. To control who can create new tables

Solution

1. Step 1: Understand the role of update policies

   Update policies define who is allowed to modify existing rows in a table.

2. Step 2: Differentiate from other policies

   Read policies control reading, insert policies control adding data, and table creation is unrelated.

3. Final Answer:

   To control who can change existing data in a table -> Option C

4. Quick Check:

   Update policy = control data changes [OK]

Hint: Update policies control data changes, not reads or inserts [OK]

Common Mistakes:

• Confusing update with read or insert policies
• Thinking update policies control table creation
• Assuming update policies allow adding new rows

2. Which keyword is used in Supabase policies to add extra conditions that must be true for an update to succeed?

easy

A. WITH CHECK

B. USING

C. ALLOW

D. WHERE

Solution

1. Step 1: Identify keywords for policy conditions

   Supabase uses USING to specify who can perform an action and WITH CHECK to add extra conditions on updates.

2. Step 2: Match keyword to extra conditions on update

   WITH CHECK ensures the new data meets certain rules before update is allowed.

3. Final Answer:

   WITH CHECK -> Option A

4. Quick Check:

   Extra update conditions = WITH CHECK [OK]

Hint: Use WITH CHECK for extra update conditions, USING for who can act [OK]

Common Mistakes:

• Confusing USING and WITH CHECK keywords
• Thinking WHERE is used in policies
• Assuming ALLOW is a valid keyword

3. Given this update policy snippet:

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

What happens if a user tries to update another user's profile?

medium

A. The update is allowed because USING only controls delete actions

B. The update is denied because WITH CHECK condition fails

C. The update is allowed because WITH CHECK is ignored

D. The update is denied because USING condition fails

Solution

1. Step 1: Understand USING condition role

   The USING clause controls who can perform the update. It checks if the user's ID matches the profile's user_id.

2. Step 2: Check condition for updating another user's profile

   If a user tries to update a different profile, auth.uid() = profiles.user_id is false, so update is denied.

3. Final Answer:

   The update is denied because USING condition fails -> Option D

4. Quick Check:

   USING controls update permission = denied if false [OK]

Hint: USING controls who can update; false means deny [OK]

Common Mistakes:

• Thinking WITH CHECK controls permission to update
• Assuming USING applies only to deletes
• Ignoring that both USING and WITH CHECK must pass

4. You wrote this delete policy:

CREATE POLICY "Delete own posts" ON posts
FOR DELETE
USING (user_id = auth.uid());

But users report they cannot delete their posts. What is the likely problem?

medium

A. The policy should use WITH CHECK instead of USING

B. The policy might be missing enabling RLS on the table

C. The policy is correct; problem is elsewhere

D. The policy is missing FOR UPDATE clause

Solution

1. Step 1: Check policy syntax

   The policy syntax is correct for delete with USING condition matching user_id.

2. Step 2: Consider RLS status

   If Row Level Security (RLS) is not enabled on the table, policies have no effect and all actions are denied by default.

3. Final Answer:

   The policy might be missing enabling RLS on the table -> Option B

4. Quick Check:

   RLS must be enabled for policies to work [OK]

Hint: Enable RLS on table to activate policies [OK]

Common Mistakes:

• Confusing USING and WITH CHECK for delete policies
• Forgetting to enable RLS on the table
• Thinking FOR UPDATE is needed for delete policies

5. You want to allow users to update their own orders only if the order status is not 'shipped'. Which policy correctly enforces this?

hard

A. FOR UPDATE USING (auth.uid() = orders.user_id) WITH CHECK (new.status != 'shipped')

B. FOR UPDATE USING (auth.uid() = orders.user_id) WITH CHECK (orders.status != 'shipped')

C. FOR UPDATE USING (auth.uid() = orders.user_id AND orders.status != 'shipped')

D. FOR UPDATE USING (auth.uid() = orders.user_id AND new.status != 'shipped')

Solution

1. Step 1: Understand USING vs WITH CHECK

   USING controls who can perform the update; WITH CHECK ensures the new data meets conditions.

2. Step 2: Apply condition on new data

   The status check must apply to the new value after update, so use new.status != 'shipped' in WITH CHECK.

3. Step 3: Verify options

   FOR UPDATE USING (auth.uid() = orders.user_id) WITH CHECK (new.status != 'shipped') correctly uses USING (auth.uid() = orders.user_id) and WITH CHECK (new.status != 'shipped').

4. Final Answer:

   FOR UPDATE USING (auth.uid() = orders.user_id) WITH CHECK (new.status != 'shipped') -> Option A

5. Quick Check:

   Use WITH CHECK with new. for update conditions [OK]

Hint: Use WITH CHECK with new. to check updated values [OK]

Common Mistakes:

• Checking old data instead of new data in WITH CHECK
• Putting status condition in USING instead of WITH CHECK
• Forgetting to use new. prefix for updated row