Introduction

As your Supabase project grows, managing permissions and access control becomes increasingly important. In this article, we'll explore advanced techniques to secure your data and ensure that users can only access the information they're supposed to.

Row Level Security (RLS) Policies

Row Level Security is a powerful feature in Supabase that allows you to define access rules at the row level. Let's dive into some advanced RLS techniques:

Complex Conditions

You can create sophisticated RLS policies using complex conditions. For example:

CREATE POLICY "Users can view their own posts and public posts"
ON posts
FOR SELECT
USING (
  auth.uid() = user_id
  OR
  (is_public = true AND published_at <= now())
);

This policy allows users to view their own posts and any public posts that have been published.

Using Functions in Policies

For more flexibility, you can create custom functions and use them in your RLS policies:

CREATE FUNCTION is_post_owner(post_row posts) RETURNS BOOLEAN AS $$
  SELECT auth.uid() = post_row.user_id;
$$ LANGUAGE sql SECURITY DEFINER;

CREATE POLICY "Users can edit their own posts"
ON posts
FOR UPDATE
USING (is_post_owner(posts.*));

This approach allows you to reuse logic across multiple policies and keeps your policy definitions clean.

Role-Based Access Control

While RLS is powerful, sometimes you need broader access control based on user roles. Here's how you can implement role-based access control in Supabase:

1. Create a roles table:

CREATE TABLE roles (
  id SERIAL PRIMARY KEY,
  name TEXT UNIQUE NOT NULL
);

INSERT INTO roles (name) VALUES ('admin'), ('editor'), ('user');

2. Add a role_id column to your users table:

ALTER TABLE auth.users
ADD COLUMN role_id INTEGER REFERENCES roles(id);

3. Create a function to check user roles:

CREATE FUNCTION get_user_role() RETURNS TEXT AS $$
  SELECT r.name
  FROM auth.users u
  JOIN roles r ON u.role_id = r.id
  WHERE u.id = auth.uid();
$$ LANGUAGE sql SECURITY DEFINER;

4. Use the function in your RLS policies:

CREATE POLICY "Admins can do anything"
ON posts
USING (get_user_role() = 'admin');

CREATE POLICY "Editors can update any post"
ON posts
FOR UPDATE
USING (get_user_role() IN ('admin', 'editor'));

Custom Claims for Fine-Grained Authorization

Supabase allows you to add custom claims to JWT tokens, which can be used for even more granular access control:

1. Enable JWT claims in your Supabase project settings.

2. Add custom claims when a user signs up or updates their profile:

const { data, error } = await supabase.auth.updateUser({
  data: { custom_claim: 'value' }
})

3. Use the custom claim in your RLS policies:

CREATE POLICY "Premium users can access exclusive content"
ON exclusive_content
FOR SELECT
USING (
  auth.jwt()->>'custom_claim' = 'premium'
);

Handling Multiple Tenants

If your application supports multiple tenants, you can implement tenant isolation using RLS:

1. Add a tenant_id column to your tables:

ALTER TABLE your_table
ADD COLUMN tenant_id UUID NOT NULL;

2. Create an RLS policy to enforce tenant isolation:

CREATE POLICY "Tenant isolation"
ON your_table
USING (tenant_id = auth.jwt()->>'tenant_id');

3. When creating or updating rows, make sure to set the tenant_id:

const { data, error } = await supabase
  .from('your_table')
  .insert({ 
    ...otherFields,
    tenant_id: user.tenant_id 
  })

Testing Your Policies

It's crucial to thoroughly test your RLS policies. You can use Supabase's auth.uid() function to simulate different user contexts:

-- Set the role to a specific user
SET LOCAL ROLE authenticated;
SET LOCAL request.jwt.claims = '{"sub": "user_id_here"}';

-- Now run your queries to test the policies
SELECT * FROM your_table;

By following these advanced techniques, you can create a robust and secure permission system in your Supabase project. Remember to always test your policies thoroughly and keep your security measures up to date as your application evolves.