Alright, let's start by creating a table to store tenant-specific information. This table will have unique identifiers for each tenant and any other necessary metadata.

create table tenants (
    id uuid primary key default uuid_generate_v4(),
    name text not null,
    -- Add any other tenant-specific fields here
);

Next, we need to modify the existing user table to include a foreign key reference to the tenants' table. This way, we can link users to their respective tenants.

alter table users
add column tenant_id uuid references tenants(id);

Now, let's enable row-level security on the tables that will use multi-tenancy. This is crucial for isolating tenant data.

alter table users enable row level security;
alter table your_data_table enable row level security;

Time to create RLS policies to ensure that tenants can only access their own data. This is where the magic happens!

For the users table:

create policy "Users can only access their own data" on users
    for select using (auth.uid() = id);

create policy "Users can insert into their own tenant" on users
    for insert with check (tenant_id = (select tenant_id from users where id = auth.uid()));

For the your_data_table:

create policy "Users only see their tenant's data" on your_data_table
    for select using (
        tenant_id = (select tenant_id from users where id = auth.uid())
    );

create policy "Users can insert their tenant's data" on your_data_table
    for insert with check (
        tenant_id = (select tenant_id from users where id = auth.uid())
    );

Ensure that the user creation process includes assigning the correct tenant ID. This can be done via a function or middleware in your backend logic.

Example: After user registration, update the user's tenant_id.

import { supabase } from './supabaseClient';

async function assignTenant(userId, tenantId) {
    const { data, error } = await supabase
        .from('users')
        .update({ tenant_id: tenantId })
        .eq('id', userId);
    
    if (error) {
        console.error('Error assigning tenant:', error);
    } else {
        console.log('Tenant assigned:', data);
    }
}

When querying data, make sure tenant-specific logic is applied. Supabase's RLS policies usually handle this automatically. Just ensure your application's client passes the correct tenant context when performing operations.

Example: Fetch user-specific data.

const { data, error } = await supabase
    .from('your_data_table')
    .select('*')
    .eq('tenant_id', logged_in_user.tenant_id);
    
if (error) {
    console.error('Error fetching tenant data:', error);
} else {
    console.log('Tenant data:', data);
}

Regularly review the RLS policies and tenant data to ensure compliance and security. Regular cuts of database logs and audits will help maintain your multi-tenant setup's health.

-- Sample audit log query (if you have audit logs setup)
select * from audit_log where action = 'SELECT' and table_name = 'your_data_table';

Keep the system up-to-date with new tenants and maintain a log of all tenant-related actions.