Multi-Tenant SaaS with Row-Level Security on Supabase

The Problem

You're building a SaaS. Multiple businesses will use the same database. Business A must never see Business B's data. Not "probably won't" — must not.

This is the multi-tenant data isolation problem.

The Solution: Row-Level Security

Supabase (PostgreSQL) has built-in Row-Level Security. Every row in every table can be filtered by who's asking.

CREATE POLICY "business_isolation" ON job_cards
  USING (business_id = (auth.jwt() ->> 'business_id')::uuid);

That one line means: every query automatically filters to only rows where the business_id matches the caller's business (read straight from their JWT). No application code needed. No middleware. No accidentally forgetting a filter.

How It Works in Practice

1. User logs in → gets a JWT carrying their business_id
2. Every query against Postgres automatically has the RLS policy applied
3. Business A queries job cards → only sees their own
4. Business A tries to read Business B's rows → empty result (not an error, just empty)

What I Built

A workshop management ERP with:

• Multi-tenant architecture
• 17 SQL migrations
• Job card management
• Invoicing & billing
• Inventory tracking
• WhatsApp integration

All running on Supabase with RLS policies enforcing data isolation.