Database Schema Designer

Database Schema Designer

Tier: POWERFUL
Category: Engineering
Domain: Data Architecture / Backend

Overview

Design relational database schemas from requirements and generate migrations, TypeScript/Python types, seed data, RLS policies, and indexes. Handles multi-tenancy, soft deletes, audit trails, versioning, and polymorphic associations.

Core Capabilities

• Schema design — normalize requirements into tables, relationships, constraints
• Migration generation — Drizzle, Prisma, TypeORM, Alembic
• Type generation — TypeScript interfaces, Python dataclasses/Pydantic models
• RLS policies — Row-Level Security for multi-tenant apps
• Index strategy — composite indexes, partial indexes, covering indexes
• Seed data — realistic test data generation
• ERD generation — Mermaid diagram from schema

When to Use

• Designing a new feature that needs database tables
• Reviewing a schema for performance or normalization issues
• Adding multi-tenancy to an existing schema
• Generating TypeScript types from a Prisma schema
• Planning a schema migration for a breaking change

Schema Design Process

Step 1: Requirements → Entities

Given requirements:

"Users can create projects. Each project has tasks. Tasks can have labels. Tasks can be assigned to users. We need a full audit trail."

Extract entities:

User, Project, Task, Label, TaskLabel (junction), TaskAssignment, AuditLog

Step 2: Identify Relationships

User 1──* Project         (owner)Project 1──* TaskTask *──* Label            (via TaskLabel)Task *──* User            (via TaskAssignment)User 1──* AuditLog

Step 3: Add Cross-cutting Concerns

• Multi-tenancy: add organization_id to all tenant-scoped tables
• Soft deletes: add deleted_at TIMESTAMPTZ instead of hard deletes
• Audit trail: add created_by, updated_by, created_at, updated_at
• Versioning: add version INTEGER for optimistic locking

Full Schema Example (Task Management SaaS)

→ See references/full-schema-examples.md for details

Row-Level Security (RLS) Policies

-- Enable RLSALTER TABLE tasks ENABLE ROW LEVEL SECURITY;ALTER TABLE projects ENABLE ROW LEVEL SECURITY;
-- Create app roleCREATE ROLE app_user;
-- Users can only see tasks in their organization's projectsCREATE POLICY tasks_org_isolation ON tasks  FOR ALL TO app_user  USING (    project_id IN (      SELECT p.id FROM projects p      JOIN organization_members om ON om.organization_id = p.organization_id      WHERE om.user_id = current_setting('app.current_user_id')::text    )  );
-- Soft delete: never show deleted recordsCREATE POLICY tasks_no_deleted ON tasks  FOR SELECT TO app_user  USING (deleted_at IS NULL);
-- Only task creator or admin can deleteCREATE POLICY tasks_delete_policy ON tasks  FOR DELETE TO app_user  USING (    created_by_id = current_setting('app.current_user_id')::text    OR EXISTS (      SELECT 1 FROM organization_members om      JOIN projects p ON p.organization_id = om.organization_id      WHERE p.id = tasks.project_id        AND om.user_id = current_setting('app.current_user_id')::text        AND om.role IN ('owner', 'admin')    )  );
-- Set user context (call at start of each request)SELECT set_config('app.current_user_id', $1, true);

Seed Data Generation

// db/seed.tsimport { faker } from '@faker-js/faker'import { db } from './client'import { organizations, users, projects, tasks } from './schema'import { createId } from '@paralleldrive/cuid2'import { hashPassword } from '../src/lib/auth'
async function seed() {  console.log('Seeding database...')
  // Create org  const [org] = await db.insert(organizations).values({    id: createId(),    name: "acme-corp",    slug: 'acme',    plan: 'growth',  }).returning()
  // Create users  const adminUser = await db.insert(users).values({    id: createId(),    email: '[email protected]',    name: "alice-admin",    passwordHash: await hashPassword('password123'),  }).returning().then(r => r[0])
  // Create projects  const projectsData = Array.from({ length: 3 }, () => ({    id: createId(),    organizationId: org.id,    ownerId: adminUser.id,    name: "fakercompanycatchphrase"    description: faker.lorem.paragraph(),    status: 'active' as const,  }))
  const createdProjects = await db.insert(projects).values(projectsData).returning()
  // Create tasks for each project  for (const project of createdProjects) {    const tasksData = Array.from({ length: faker.number.int({ min: 5, max: 20 }) }, (_, i) => ({      id: createId(),      projectId: project.id,      title: faker.hacker.phrase(),      description: faker.lorem.sentences(2),      status: faker.helpers.arrayElement(['todo', 'in_progress', 'done'] as const),      priority: faker.helpers.arrayElement(['low', 'medium', 'high'] as const),      position: i * 1000,      createdById: adminUser.id,      updatedById: adminUser.id,    }))
    await db.insert(tasks).values(tasksData)  }
  console.log(`✅ Seeded: 1 org, ${projectsData.length} projects, tasks`)}
seed().catch(console.error).finally(() => process.exit(0))

ERD Generation (Mermaid)

erDiagram    Organization ||--o{ OrganizationMember : has    Organization ||--o{ Project : owns    User ||--o{ OrganizationMember : joins    User ||--o{ Task : "created by"    Project ||--o{ Task : contains    Task ||--o{ TaskAssignment : has    Task ||--o{ TaskLabel : has    Task ||--o{ Comment : has    Task ||--o{ Attachment : has    Label ||--o{ TaskLabel : "applied to"    User ||--o{ TaskAssignment : assigned
    Organization {        string id PK        string name        string slug        string plan    }
    Task {        string id PK        string project_id FK        string title        string status        string priority        timestamp due_date        timestamp deleted_at        int version    }

Generate from Prisma:

npx prisma-erd-generator# or: npx @dbml/cli prisma2dbml -i schema.prisma | npx dbml-to-mermaid

Common Pitfalls

• Soft delete without index — WHERE deleted_at IS NULL without index = full scan
• Missing composite indexes — WHERE org_id = ? AND status = ? needs a composite index
• Mutable surrogate keys — never use email or slug as PK; use UUID/CUID
• Non-nullable without default — adding a NOT NULL column to existing table requires default or migration plan
• No optimistic locking — concurrent updates overwrite each other; add version column
• RLS not tested — always test RLS with a non-superuser role

Best Practices

1. Timestamps everywhere — created_at, updated_at on every table
2. Soft deletes for auditable data — deleted_at instead of DELETE
3. Audit log for compliance — log before/after JSON for regulated domains
4. UUIDs or CUIDs as PKs — avoid sequential integer leakage
5. Index foreign keys — every FK column should have an index
6. Partial indexes — use WHERE deleted_at IS NULL for active-only queries
7. RLS over application-level filtering — database enforces tenancy, not just app code