Update and delete policies in Supabase - Mini Project: Build & Apply

Update and delete policies

📖 Scenario: You are managing a simple database for a small online store using Supabase. You want to control who can update or delete product records to keep your data safe.

🎯 Goal: Create policies in Supabase that allow only the product owner to update or delete their product entries.

📋 What You'll Learn

Create a table called products with columns id, owner_id, and name

Add a policy to allow updates only if the user owns the product

Add a policy to allow deletes only if the user owns the product

💡 Why This Matters

🌍 Real World

Controlling who can change or remove data is important for security in any app that stores user data.

💼 Career

Understanding update and delete policies is key for database administrators and backend developers working with secure cloud databases.

1

Create the products table

Write the SQL command to create a table called products with columns id as a primary key, owner_id as UUID, and name as text.

Supabase

-- Your SQL command here

Hint

Use create table with the specified columns and types.

2

Add update policy for product owners

Write the SQL command to create a policy named Allow update if owner on the products table that allows updates only when auth.uid() equals owner_id.

Supabase

create table products (
  id serial primary key,
  owner_id uuid not null,
  name text not null
);
-- Your SQL command here

Hint

Use create policy with for update and a using condition comparing auth.uid() to owner_id.

3

Add delete policy for product owners

Write the SQL command to create a policy named Allow delete if owner on the products table that allows deletes only when auth.uid() equals owner_id.

Supabase

create table products (
  id serial primary key,
  owner_id uuid not null,
  name text not null
);

create policy "Allow update if owner" on products
  for update
  using (auth.uid() = owner_id);
-- Your SQL command here

Hint

Similar to update policy, but use for delete.

4

Enable row level security on products

Write the SQL command to enable row level security on the products table.

Supabase

create table products (
  id serial primary key,
  owner_id uuid not null,
  name text not null
);

create policy "Allow update if owner" on products
  for update
  using (auth.uid() = owner_id);

create policy "Allow delete if owner" on products
  for delete
  using (auth.uid() = owner_id);
-- Your SQL command here

Hint

Use alter table products enable row level security; to activate policies.

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