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Start with relationships, not a global role label
A marketplace request has an identity: the signed-in user represented by auth.uid(). That identity alone does not establish which school the user belongs to, whether they may manage a particular listing, or whether they are a participant in a transaction. Supabase’s Row Level Security documentation describes policies as rules attached to tables that constrain which rows a request can access.
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Before writing policies, verify the actual role names, tables, and rules in the application’s existing schema or earlier project implementation. The names below are illustrative, not claims about this marketplace’s established schema:
- Identity: the requesting user, usually compared with
(select auth.uid()). - Membership or role: the user’s membership and permissions, potentially scoped to a specific school.
- Row scope: the school, listing owner, or transaction participants that determine which records the permission applies to.
- Table grant: whether the database role may attempt an operation such as
SELECTorUPDATE.
A school administrator role is not meaningful as a row-level rule until the database can determine which school that role applies to. Likewise, being signed in does not make a user a member of every school or a participant in every transaction. Model those relationships explicitly.
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Decide who can do what to each table
For every table exposed to the API, decide whether anonymous and authenticated requests should be able to reach it, then align its privileges and policies with that decision. Write policies for the operations the application actually needs rather than treating “has access” as one blanket permission.
| Record type | Useful row-scope question | Example policy intent |
|---|---|---|
| School membership | Which users may see or manage this membership? | A user may see their own membership; authorized school staff may manage memberships for their school. |
| Listing | Which school does it belong to, and who owns it? | A member may see listings shared with that school; a seller may manage only listings they own, subject to the school’s rules. |
| Transaction | Who are the parties to this transaction? | Only its buyer, seller, and any explicitly authorized school role may access it. |
Those are design prompts, not universal product rules. For example, if listings are public to unauthenticated visitors, define that as a deliberate anonymous-access policy rather than assuming membership is always required. Keep each policy consistent with the product’s actual sharing and moderation rules.
Separate table grants from row policies
A grant answers whether a database role can attempt an operation on a table. An RLS policy answers which rows that operation may affect. A policy does not revoke a grant that already exists, and enabling RLS does not mean every intended permission has automatically been granted. Supabase recommends reviewing privileges as well as policies.
For example, the following is only a starting point for a table that should be readable and writable by signed-in clients, but not anonymous clients. Adjust the operations to match the application, review existing privileges, and apply the same review to every exposed table:
revoke all on public.listings from anon;
revoke all on public.listings from authenticated;
grant select, insert, update, delete
on public.listings to authenticated;
alter table public.listings enable row level security;The grant permits the role to attempt those four operations; the policies that follow decide which listing rows and changes are allowed. Do not grant an operation merely because a policy exists for it. Also check default privileges and any grants already present in the project.
Rank #2
Scope policies to membership, ownership, and participation
A policy should express the relationship that authorizes access, not just a broad label such as “seller” or “admin.” Consider a listings table with school_id and seller_id, plus a membership table with school_id, user_id, and an active-membership indicator. The following examples illustrate a basic shape; adapt the columns, role distinctions, and sharing rules to the real schema.
Read listings shared with a school
create policy "school members can read listings"
on public.listings
for select
to authenticated
using (
private.is_active_school_member(school_id)
);The membership check scopes the decision to the school attached to each listing. If some listings are private to their owner or visible only to a subset of members, encode that additional condition rather than broadening the rule to all school members.
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Insert only under the caller’s identity and an authorized school
create policy "members can create their own listings"
on public.listings
for insert
to authenticated
with check (
seller_id = (select auth.uid())
and private.is_active_school_member(school_id)
);WITH CHECK constrains the proposed row. Checking only that a caller is signed in would not stop them from submitting a listing with someone else’s seller_id or attaching it to a school where they have no membership.
Keep ownership stable when updating
create policy "owners can update their listings"
on public.listings
for update
to authenticated
using (
seller_id = (select auth.uid())
and private.is_active_school_member(school_id)
)
with check (
seller_id = (select auth.uid())
and private.is_active_school_member(school_id)
);For an update, USING limits which existing rows are eligible, while WITH CHECK validates the resulting row. Requiring the resulting seller_id to remain the caller prevents an owner from transferring the row’s ownership through an update. Supabase notes that an UPDATE also needs a matching SELECT policy to behave as expected, so test both together.
If the product permits transfers, implement that as a separate, deliberate authorization flow rather than weakening the ordinary owner-update rule. Consider whether fields such as school_id, seller identity, transaction status, or payment state must be immutable to client updates.
Rank #3
Limit transaction access to its actual participants
For a transaction table with buyer and seller user IDs, a participant-scoped read rule could look like this:
create policy "transaction participants can read"
on public.transactions
for select
to authenticated
using (
buyer_id = (select auth.uid())
or seller_id = (select auth.uid())
);Creation and status changes need their own rules. A buyer should not be able to create a transaction naming an unrelated buyer, and a participant should not automatically be allowed to change every transaction field. Define which party may initiate, cancel, or advance each state, and validate both the existing row and the proposed row for updates.
Choose where role and membership data lives
There are two common patterns: carry selected role or permission claims in a signed JWT, or look up current membership and role data in the database. They can be combined, but they answer different needs. Supabase’s Custom Claims & Role-based Access Control guide demonstrates adding a role claim through an access-token hook and checking claims in policies.
| Decision factor | JWT role or permission claims | Database-backed membership or role |
|---|---|---|
| School-specific relationships | Useful for claims that are stable and compact; a single global role does not express different permissions at different schools. School-specific claims require an explicit design. | Can represent separate memberships, roles, and statuses for each school directly. |
| Updates and freshness | The claim is derived from a trusted source and is carried in an access token. Plan how changes reach newly issued or refreshed tokens; do not assume a role change rewrites an already issued token immediately. | Policies can consult the current membership row, subject to the application’s transaction and caching design. |
| Policy complexity | Can make checks concise, but claim creation, hook configuration, and trust in the claim source are part of the security boundary. | Expresses relational checks in the database, but policy queries may create recursion or require a carefully designed helper. |
| Privileged code | Depends on secure claim issuance and verification. | A helper that bypasses RLS can introduce elevated privileges and must be narrowly constrained. |
Choose based on how often permissions change, whether they vary by school, and where the authoritative role data belongs. A JWT claim is not trustworthy merely because it has a role-shaped name: it must come from a trusted issuance path. A database lookup is not automatically safer; its policies and any privileged helper need review too.
Prevent membership-policy recursion carefully
If a policy on a membership table queries that same table to decide access, the policy may recursively invoke itself. Supabase documents a private security definer helper as one way to break this recursion. A definer function runs with its owner’s privileges, so its owner, search path, SQL, exposure, and execute grants all matter.
An illustrative helper pattern is:
create schema if not exists private;
create function private.is_active_school_member(_school_id uuid)
returns boolean
language sql
stable
security definer
set search_path = ''
as $$
select exists (
select 1
from public.school_memberships as sm
where sm.school_id = _school_id
and sm.user_id = (select auth.uid())
and sm.is_active = true
);
$$;
revoke all on function private.is_active_school_member(uuid) from public;
grant execute on function private.is_active_school_member(uuid)
to authenticated;This example assumes the named schema, columns, and active flag exist; it is not a drop-in function for an unknown project. Keep such helpers in a non-exposed schema, use schema-qualified table references and an empty pinned search_path, and restrict execution to the roles that need it. Ensure the function owner has only the privileges needed for the check, and verify that the function uses the requesting user’s identity rather than an unchecked user ID supplied by the caller.
Prefer security invoker for ordinary database functions when they do not need elevated privileges. If a security definer function is necessary, Supabase’s Database functions guidance covers the relevant security considerations. Review every function exposed to API roles and grant EXECUTE narrowly.
Review views, columns, and keys beyond the policy itself
Views can change the RLS boundary
Supabase notes that views can bypass underlying RLS by default because they are typically created by postgres. On Postgres 15 and later, a view can be created with security_invoker = true so the underlying table policies apply to anon and authenticated callers. Review who owns each view, what it exposes, and which roles can query it; do not assume a protected base table makes every view safe.
RLS controls rows, not individual columns
A row policy does not decide which columns within an allowed row a user may update. If clients must edit only selected fields, consider whether the update should go through a narrowly scoped function or whether column privileges are appropriate. Supabase labels column-level security an advanced feature and generally recommends RLS with a dedicated role table for ordinary role-based access needs; see its Column Level Security guidance.
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Keep privileged keys off client devices
Supabase’s data security guidance states that secret and service-role keys bypass RLS and belong only on a trusted backend. A publishable key may be used in a frontend only when RLS and least-privilege grants are configured. Treat key exposure as a separate risk from policy correctness: a well-written policy cannot protect rows from a key that bypasses it.
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Test both permitted and forbidden paths
Write database tests for every exposed table and each operation the client can attempt. Supabase recommends supabase test db and tests that assert both allow and deny behavior. For a school marketplace, test not only a happy-path owner but also the users and tenants most likely to reveal an overbroad rule.
- Anonymous requests, wherever the table is reachable without sign-in.
- Signed-in users with no relevant membership.
- An active school member who does not own the listing.
- The listing owner, including a request that tries to change its owner or school.
- A member of a second school attempting to access the first school’s records.
- A transaction participant and a non-participant.
- Attempts to insert rows with another user’s identity, an unauthorized school, or unrelated transaction participants.
- Attempts to update protected fields, read rows outside scope, or delete records the caller should not manage.
For each case, test SELECT, INSERT, UPDATE, and DELETE as applicable. Test grants independently: a denied row policy does not establish that the grant configuration is least-privilege, and a missing grant can make an intended policy path unreachable. Also cover views and functions that expose data or perform writes.
Supabase’s Row Level Security documentation puts the value of verification plainly: “Until the suite passes, you don’t know whether the policies do what you intended.”
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Optimize only after the access rules are correct
Supabase suggests wrapping fixed JWT or helper calls in a scalar subquery—for example, (select auth.uid())—so Postgres can evaluate and cache the result per statement rather than per row. It also recommends indexing columns used to filter policies, such as user, school, and participant identifiers. These are optimization considerations, not a guarantee of faster queries: measure the actual workload and query plans before attributing a performance change to either choice.
Keep correctness tests as a gate when changing policy expressions, membership helpers, indexes, claims, grants, or exposed views. A fast policy that grants the wrong school access is a failed policy.
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