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SaaS with Supabase

Experiment #5417th March, 2021by Joshua Nussbaum

I’m going to use Supabase for some SaaS style projects.

In supabase, tables can be protected with RLS (row-level security), typically to check if the user is an owner:

-- enable RLS
alter table products enable row level security;

-- add a policy
create policy "Can view own data." on products for select using (auth.uid() = id);

But what if want to check is the user has paid?

We can create a shadow users table to keep track of active users. Why a shadow table? Because auth.users is controlled by Supabase, we should avoid clashing with that and keep our data in the public schema along with the rest of our data.

Here’s how we shadow the users table:

-- create a public.users table
create table users (
  id uuid references auth.users not null primary key,
  active boolean not null default false

-- enable RLS
alter table users enable row level security;

-- add access policies
create policy "Can view own user data." on users for select using (auth.uid() = id);
create policy "Can update own user data." on users for update using (auth.uid() = id);

We’ll also need a trigger to create a record in public.users anytime a record is added to auth.users:

create or replace function public.handle_new_user() 
returns trigger as $$
  insert into public.users (id, active)
  values (new.id, false);
  return new;
$$ language plpgsql security definer;

create trigger on_auth_user_created
  after insert on auth.users
  for each row execute procedure public.handle_new_user();

Now if we want restrict data to paid members only, we can check if user.active bit is enabled.

That’s going to happen a lot, so let’s create a reusable function we can use in all our policies:

create or replace function is_paid_member(uid uuid)
returns boolean as $$
declare is_paid boolean;
  select active into is_paid from users where id = uid;
  return coalesce(is_paid, false);
end $$ language plpgsql stable security definer;

Here’s how we’d use it:

Let’s say only paid members can save their “favorite colors”. Create a table to track that data with user_id, and let’s add some policies. We’ll check with is_paid_member() where appropriate.

-- create table with restricted data
create table colors (
  id serial primary key,
  user_id uuid references users(id),
  value text not null

-- enable RLS
alter table colors enable row level security;

-- setup access policies
create policy "can read own data" on colors for select using (auth.uid() = user_id);
create policy "can insert own data when is a paid member" on colors for insert with check (auth.uid() = user_id and is_paid_member(user_id));
create policy "can update own data when is a paid member" on colors for update using (auth.uid() = user_id and is_paid_member(user_id));
create policy "can delete own data when is a paid member" on colors for delete using (auth.uid() = user_id and is_paid_member(user_id));

In this case, we allow unpaid members read access to their data, but insert, update & delete must be paid.


Once we create a reusable is_paid_member() function, it’s easy to restrict data. In the future, I’ll look into flipping that bit thru Stripe.

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