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Secure a school marketplace by deciding four things separately: who is making each request, what role or membership that person has, which rows that authority covers, and whether the database role has permission to attempt the operation. Supabase Row Level Security (RLS) can enforce row access, but it does not replace table grants, safe function design, or tests. Because the exact schema and role names vary by project, treat the examples below as patterns to adapt—not facts about a particular marketplace.
Separate identity, authority, row scope, and grants
These checks answer different questions and should be designed together:
- Identity: Which signed-in user is making the request? In Supabase policies,
auth.uid()can identify the authenticated user. - Role or membership: What may that user do? A user might be a buyer, seller, or school administrator, but the label alone may not say which school or records the authority applies to.
- Row scope: Which listings, memberships, or transactions may the user access? Model relationships such as listing ownership, school membership, and transaction participation explicitly.
- Table grants: May the database role attempt a given operation on the table at all? A policy filters rows; it does not revoke a grant that already exists.
Supabase describes RLS policies as rules attached to tables that constrain which rows are available to a request. Its Row Level Security documentation uses auth.uid() to scope access to the authenticated user. For a marketplace, that identity check is only one part of the rule: the policy must also reflect the relevant school and the user’s relationship to the specific record.
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A global role can work when authority is genuinely global. When permissions differ by school, store or derive that school-specific relationship explicitly. Two common approaches have different trade-offs:
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| Approach | Useful when | Trade-offs to review |
|---|---|---|
| Custom role or permission claims in a JWT | Policies need to check trusted claims included in the user’s access token. | Claims must come from a trusted source, such as an access-token hook. Changes to roles or permissions are not reflected in an already-issued token until a fresh token is issued; plan for that update lifecycle. A broad role claim also does not by itself express which school a user belongs to. |
| Database-backed membership and role data | Permissions depend on relationships such as a user’s membership in a particular school. | Policies can express tenant-specific relationships, but membership lookups add policy complexity. If a policy queries a table whose own policies refer back to the original policy, recursion can occur; handle that deliberately rather than weakening access checks. |
Supabase’s custom claims and RBAC guide describes adding a role claim with an access-token hook and checking permissions in policies. Use a source of role data that only trusted systems can change. A claim is evidence of authority only if its creation and refresh lifecycle are controlled.
For database-backed membership checks, Supabase documents a private security definer helper pattern that can break policy recursion. Such a helper runs with its owner’s privileges, so use it only after reviewing its boundaries: pin an empty search_path, schema-qualify referenced objects, verify the caller or ownership as appropriate, and grant execution narrowly. Do not treat a helper as a harmless shortcut.
Set table grants before relying on row policies
For every exposed table, decide whether anonymous and authenticated clients should have access, then grant only the operations each client role needs. Review existing project defaults rather than assuming they are restrictive. Enable RLS and create operation-specific policies for SELECT, INSERT, UPDATE, and DELETE where those operations are intended.
The distinction matters in both directions: a policy cannot grant an operation the database role lacks permission to perform, and a policy does not cancel a table grant. Supabase’s data security guidance also makes key handling part of the boundary: secret and service-role keys bypass RLS and belong on the backend only. A publishable key may be used in a frontend only when RLS and least-privilege grants are configured.
Scope each operation to the right marketplace relationship
Write policies around the records and relationships the product actually supports. For example, a listings table might distinguish public browsing from seller editing; a transactions table might limit access to its buyer, seller, or another explicitly authorized participant; and school-managed records might require membership in the record’s school. Do not assume a single marketplace-wide role is sufficient to answer all three questions.
This illustrative policy shows the shape of an ownership check, not a complete school-marketplace schema. Replace the example table and column names with the real schema and add any school-membership conditions the product requires:
alter table public.listings enable row level security;
grant select, insert, update, delete
on public.listings to authenticated;
create policy "seller reads own listings"
on public.listings for select to authenticated
using (owner_id = (select auth.uid()));
create policy "seller inserts own listings"
on public.listings for insert to authenticated
with check (owner_id = (select auth.uid()));
create policy "seller updates own listings"
on public.listings for update to authenticated
using (owner_id = (select auth.uid()))
with check (owner_id = (select auth.uid()));
create policy "seller deletes own listings"
on public.listings for delete to authenticated
using (owner_id = (select auth.uid()));The grant in the example is intentionally separate from the policies: it lets the authenticated database role attempt those operations, while the policies restrict the affected rows. In a real application, omit operations the client does not need. Public listing reads, if intended, need a separate decision about anonymous access and the rows that should be visible to anonymous users.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsFor writes, reason about both the existing row and the resulting row. USING limits which existing rows an update or delete can affect. WITH CHECK constrains inserted rows and the new version of an updated row. In the example, checking owner_id after an update prevents a seller from transferring ownership by changing that field. Supabase notes that an UPDATE also needs a matching SELECT policy to behave as expected.
Membership-based rules need tests for both a member who does not own a record and a non-member. That distinction catches policies that accidentally treat membership as ownership or allow access across school boundaries.
Review views, functions, and column-level access
RLS on a base table is not the end of the review. Views and database functions can expose or modify protected data through different privilege paths. Supabase notes that views can bypass RLS by default because they are usually created with postgres. On PostgreSQL 15 and later, a view can use security_invoker = true so underlying RLS applies to anon and authenticated callers. Check the version and security behavior of each view rather than assuming it inherits the caller’s access rules.
Prefer security invoker for database functions where it fits. If a function needs security definer, keep its authority narrow: use an empty pinned search_path, schema-qualified references, caller or ownership checks, and limited EXECUTE grants. Supabase’s database functions guidance covers these function-security considerations.
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RLS decides which rows a caller can access; it does not by itself decide which columns the caller may update. Column-level privileges can help when a client should update selected fields, but Supabase describes them as an advanced feature and generally recommends RLS with a dedicated role table for ordinary access-control needs. See its Column Level Security guide before adding that extra privilege layer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test both permitted and denied access
Write SQL policy tests for each exposed table and each operation. Supabase recommends asserting both allow and deny behavior and provides supabase test db as the command to run database tests. Include cases appropriate to the product:
- Anonymous callers, where anonymous access is possible.
- Signed-in users who own a record and signed-in users who do not.
- School members who are not owners, and users who are not members.
- A user or record associated with a second school or tenant, to check that access does not cross that boundary.
- Inserts that try to assign a different owner or school, and updates that try to change immutable ownership fields.
- Expected allow and deny results for
SELECT,INSERT,UPDATE, andDELETEwherever the operation is exposed.
Test grants independently from policies, and review views, function execution grants, secret-key use, and protected columns. Supabase’s documentation puts the value of this step plainly: “Until the suite passes, you don’t know whether the policies do what you intended.”
Optimize policy checks only after they are correct
Supabase suggests wrapping fixed JWT or helper calls in a scalar subquery, such as (select auth.uid()), so Postgres can evaluate or cache the result per statement rather than per row. It also recommends indexing columns used to filter policies. These are tuning techniques, not evidence of a particular speed improvement: measure the workload and verify behavior after applying them. Correct row scope and deny cases come first.
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