Why should you care?

One product serves thousands of companies profitably — but a single missing filter is a data breach.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.tenants.insertMany([
  { _id: "acme", name: "Acme Foods", plan: "enterprise" },
  { _id: "globex", name: "Globex", plan: "pro" }
])
db.tasks.insertMany([
  { tenantId: "acme", title: "Inventory sync", status: "open" },
  { tenantId: "globex", title: "Onboard region", status: "open" }
])
db.tasks.createIndex({ tenantId: 1, status: 1, updatedAt: -1 })
function tasksFor(tenantId) {
  return db.tasks.find({ tenantId, status: "open" })
}
tasksFor("acme")

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• Two tenants share tasks collection but never share query results if tenantId is mandatory.
• A helper forces the filter.
• Unique business keys should include tenantId too.

Practice next

1. Seed two tenants and tasks.
2. build tasksFor and test isolation.
3. Add unique index { tenantId: 1, title: 1 } if titles must be unique per tenant.
4. Move a huge tenant to its own database for isolation.
5. Add tenantId to a change stream filter for per-tenant workers.

Remember

Shared collections + tenantId filters. Indexes start with tenantId. Authn derives tenant, always.

Project management SaaS

Acme never sees Globex tasks because APIs inject tenant from JWT.

Outcome: Security questionnaire on isolation gets a clear yes.