Why Update and delete policies in Supabase? - Purpose & Use Cases

The Big Idea

What if a simple rule could stop accidental data loss and keep your app safe without extra work?

The Scenario

Imagine you have a shared online notebook where many friends can write and erase notes. Without clear rules, anyone might accidentally delete or change important notes, causing confusion and lost information.

The Problem

Manually checking who can update or delete each note is slow and risky. Mistakes happen easily, like deleting the wrong note or letting someone change notes they shouldn't. This wastes time and can break trust.

The Solution

Update and delete policies set clear, automatic rules about who can change or remove data. These rules run behind the scenes, so only the right people can make updates or deletions, keeping data safe and organized without extra effort.

Before vs After

✗ Before

Allow all users to update or delete any record without checks

✓ After

Create policies that let users update or delete only their own records

What It Enables

It enables secure, automatic control over who can change or remove data, protecting your app's information effortlessly.

Real Life Example

In a task app, update and delete policies ensure users can only edit or remove their own tasks, preventing others from messing with their work.

Key Takeaways

Manual control over updates and deletes is slow and error-prone.

Policies automate who can change or remove data safely.

This keeps your app's data secure and trustworthy.

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