Update and delete policies in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of update and delete policies in Supabase?

They control who can change or remove data in your database, ensuring only authorized users can perform these actions.

beginner

How do you write a simple update policy in Supabase?

You write a policy using SQL-like expressions that check conditions, for example: auth.uid() = user_id to allow users to update only their own data.

beginner

Why is it important to have delete policies in your database?

Delete policies prevent accidental or malicious removal of data by restricting who can delete records, protecting data integrity.

beginner

What happens if you do not set update or delete policies in Supabase?

By default, no one can update or delete data unless policies explicitly allow it, so your data remains safe from unauthorized changes.

intermediate

Can update and delete policies use multiple conditions in Supabase?

Yes, you can combine conditions using AND/OR to create precise rules, like allowing updates only if the user owns the record and the record is active.

What does an update policy in Supabase control?

A Who can create new data

B Who can read data

C Who can change existing data

D Who can backup data

Update policies define who is allowed to modify existing records in the database.

If no delete policy is set, who can delete data in Supabase?

A No one

B Only admins

C Only the data owner

D Anyone

By default, no delete operations are allowed unless a policy explicitly permits it.

Which function is commonly used in Supabase policies to check the current user ID?

A auth.uid()

B current_user()

C user_id()

D session.user()

The function auth.uid() returns the ID of the authenticated user for policy checks.

Can you combine multiple conditions in a Supabase update policy?

A No, only one condition is allowed

B Yes, using AND/OR operators

C Only OR is allowed

D Only AND is allowed

Supabase policies support combining multiple conditions with AND and OR for precise control.

What is a best practice when creating delete policies?

A Allow delete only during business hours

B Allow everyone to delete

C Disable delete policies

D Restrict delete to authorized users only

Restricting delete operations to authorized users protects data from accidental or malicious loss.

Explain how update policies work in Supabase and why they are important.

Think about who should be allowed to change data and how Supabase checks user identity.

Describe the default behavior of Supabase when no delete policies are set and how to properly secure delete operations.

Consider what happens if you forget to set delete rules and how to prevent unwanted data loss.

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