Update and delete policies in Supabase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allow users to update their own profiles.

Supabase

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

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

Auser_id

Bowner_id

Cprofile_id

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field that does not uniquely identify the user.

Using a field that does not exist in the table.

The policy checks if the authenticated user's ID matches the user_id field in the profiles table to allow updates.

2fill in blank

medium

Complete the code to allow users to delete only their own posts.

Supabase

create policy "Users can delete their own posts" on posts for delete using (auth.uid() = [1]);

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

Auser_id

Bauthor

Cpost_owner

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field that does not match the authenticated user ID.

Using a generic field name that is not in the table.

The policy uses the user_id field to verify ownership before allowing deletion.

3fill in blank

hard

Fix the error in the delete policy to correctly restrict deletion to the owner.

Supabase

create policy "Delete own comments" on comments for delete using (auth.uid() = [1]);

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

Auser

Bcomment_id

Ccommenter_id

Downer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the comment's ID instead of the user ID.

Using a field that does not exist in the comments table.

The correct field to check ownership is commenter_id, which stores the user who made the comment.

4fill in blank

hard

Fill both blanks to create a policy that allows users to update their own orders only if the order status is 'pending'.

Supabase

create policy "Update own pending orders" on orders for update using (auth.uid() = [1] and status = [2]);

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

Acustomer_id

B'pending'

C'completed'

Duser_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong user field name.

Using the wrong status string.

The policy checks that the authenticated user matches customer_id and that the order status is exactly 'pending' before allowing updates.

5fill in blank

hard

Fill all three blanks to create a delete policy that allows users to delete their own messages only if the message is unread and the user is the recipient.

Supabase

create policy "Delete own unread messages" on messages for delete using (auth.uid() = [1] and status = [2] and recipient_id = [3]);

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

Asender_id

B'unread'

Cauth.uid()

Drecipient_id

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up sender and recipient fields.

Using incorrect status values.

Not matching recipient to authenticated user.

The policy ensures the authenticated user is the recipient (recipient_id), the message status is 'unread', and the authenticated user matches the recipient_id to allow deletion.

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