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Atlas Whoff
Atlas Whoff

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Multi-Tenant SaaS Architecture: Row-Level Tenancy, RBAC, and Org-Scoped Queries

#saas #database #typescript #webdev

Building a multi-tenant SaaS means every piece of data must be scoped to the right organization. Get this wrong and you have a catastrophic data leak. Get it right and you have a foundation that scales to thousands of tenants.

Tenancy Models

Row-level tenancy (recommended for most SaaS): All tenants share the same database. Every table has an organizationId column. Queries filter by it.

Schema-per-tenant: Each tenant gets a separate Postgres schema. More isolation, more operational complexity.

Database-per-tenant: Maximum isolation. Reserved for enterprise tiers with strict compliance requirements.

Row-level tenancy handles 99% of use cases. Start here.

Schema Design 

model Organization {
  id        String   @id @default(cuid())
  name      String
  slug      String   @unique
  plan      String   @default("free")
  createdAt DateTime @default(now())

  members   OrganizationMember[]
  projects  Project[]
  invoices  Invoice[]
}

model OrganizationMember {
  id             String       @id @default(cuid())
  organizationId String
  userId         String
  role           String       @default("member")  // owner, admin, member
  organization   Organization @relation(fields: [organizationId], references: [id])
  user           User         @relation(fields: [userId], references: [id])

  @@unique([organizationId, userId])
}

model Project {
  id             String       @id @default(cuid())
  organizationId String
  name           String
  organization   Organization @relation(fields: [organizationId], references: [id])

  @@index([organizationId])  // critical for query performance
}
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Resolving the Current Organization 

// lib/org.ts
export async function getCurrentOrg(userId: string, orgSlug: string) {
  const membership = await db.organizationMember.findFirst({
    where: {
      userId,
      organization: { slug: orgSlug },
    },
    include: { organization: true },
  })

  if (!membership) {
    throw new NotFoundError('Organization', orgSlug)
  }

  return { org: membership.organization, role: membership.role }
}
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Tenant-Scoped Queries

Every data access function takes organizationId explicitly:

// repositories/projects.ts
export async function getProjects(organizationId: string) {
  return db.project.findMany({
    where: { organizationId },  // always scope by org
    orderBy: { createdAt: 'desc' },
  })
}

export async function getProject(id: string, organizationId: string) {
  const project = await db.project.findFirst({
    where: { id, organizationId },  // both conditions required
  })

  if (!project) throw new NotFoundError('Project', id)
  return project
}
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Never query by ID alone. Always include organizationId in the WHERE clause.

URL Structure 

/[orgSlug]/dashboard          -- org home
/[orgSlug]/projects           -- project list
/[orgSlug]/projects/[id]      -- project detail
/[orgSlug]/settings           -- org settings
/[orgSlug]/settings/members   -- member management
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// app/[orgSlug]/layout.tsx
export default async function OrgLayout({
  children,
  params,
}: {
  children: React.ReactNode
  params: { orgSlug: string }
}) {
  const session = await getServerSession(authOptions)
  if (!session) redirect('/auth/signin')

  const { org, role } = await getCurrentOrg(session.user.id, params.orgSlug)

  return (
    <OrgProvider org={org} role={role}>
      {children}
    </OrgProvider>
  )
}
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Role-Based Access Control 

const PERMISSIONS = {
  'project:create': ['owner', 'admin'],
  'project:delete': ['owner'],
  'member:invite': ['owner', 'admin'],
  'member:remove': ['owner'],
  'billing:manage': ['owner'],
} as const

type Permission = keyof typeof PERMISSIONS

export function can(role: string, permission: Permission): boolean {
  return (PERMISSIONS[permission] as readonly string[]).includes(role)
}

// Usage
if (!can(membership.role, 'project:delete')) {
  throw new ForbiddenError('Only owners can delete projects')
}
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