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──* Task
Task *──* 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)

Prisma Schema

Read the detailed procedure and examples when working on this part of the task.

Drizzle Schema (TypeScript)

// db/schema.ts
import {
  pgTable, text, timestamp, integer, boolean,
  varchar, jsonb, real, pgEnum, uniqueIndex, index,
} from 'drizzle-orm/pg-core'
import { createId } from '@paralleldrive/cuid2'

export const taskStatusEnum = pgEnum('task_status', [
  'todo', 'in_progress', 'in_review', 'done', 'cancelled'
])
export const priorityEnum = pgEnum('priority', ['low', 'medium', 'high', 'critical'])

export const tasks = pgTable('tasks', {
  id:          text('id').primaryKey().$defaultFn(() => createId()),
  projectId:   text('project_id').notNull().references(() => projects.id),
  title:       varchar('title', { length: 500 }).notNull(),
  description: text('description'),
  status:      taskStatusEnum('status').notNull().default('todo'),
  priority:    priorityEnum('priority').notNull().default('medium'),
  dueDate:     timestamp('due_date', { withTimezone: true }),
  position:    real('position').notNull().default(0),
  version:     integer('version').notNull().default(1),
  createdById: text('created_by_id').notNull().references(() => users.id),
  updatedById: text('updated_by_id').notNull().references(() => users.id),
  createdAt:   timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
  updatedAt:   timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
  deletedAt:   timestamp('deleted_at', { withTimezone: true }),
}, (table) => ({
  projectIdx:       index('tasks_project_id_idx').on(table.projectId),
  projectStatusIdx: index('tasks_project_status_idx').on(table.projectId, table.status),
}))

// Infer TypeScript types
export type Task = typeof tasks.$inferSelect
export type NewTask = typeof tasks.$inferInsert

Alembic Migration (Python / SQLAlchemy)

# alembic/versions/20260301_create_tasks.py
"""Create tasks table

Revision ID: a1b2c3d4e5f6
Revises: previous_revision
Create Date: 2026-03-01 12:00:00
"""

from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects import postgresql

revision = 'a1b2c3d4e5f6'
down_revision = 'previous_revision'


def upgrade() -> None:
    # Create enums
    task_status = postgresql.ENUM(
        'todo', 'in_progress', 'in_review', 'done', 'cancelled',
        name='task_status'
    )
    task_status.create(op.get_bind())
    
    op.create_table(
        'tasks',
        sa.Column('id', sa.Text(), primary_key=True),
        sa.Column('project_id', sa.Text(), sa.ForeignKey('projects.id'), nullable=False),
        sa.Column('title', sa.VARCHAR(500), nullable=False),
        sa.Column('description', sa.Text()),
        sa.Column('status', postgresql.ENUM('todo', 'in_progress', 'in_review', 'done', 'cancelled', name='task_status', create_type=False), nullable=False, server_default='todo'),
        sa.Column('priority', sa.Text(), nullable=False, server_default='medium'),
        sa.Column('due_date', sa.TIMESTAMP(timezone=True)),
        sa.Column('position', sa.Float(), nullable=False, server_default='0'),
        sa.Column('version', sa.Integer(), nullable=False, server_default='1'),
        sa.Column('created_by_id', sa.Text(), sa.ForeignKey('users.id'), nullable=False),
        sa.Column('updated_by_id', sa.Text(), sa.ForeignKey('users.id'), nullable=False),
        sa.Column('created_at', sa.TIMESTAMP(timezone=True), nullable=False, server_default=sa.text('NOW()')),
        sa.Column('updated_at', sa.TIMESTAMP(timezone=True), nullable=False, server_default=sa.text('NOW()')),
        sa.Column('deleted_at', sa.TIMESTAMP(timezone=True)),
    )
    
    # Indexes
    op.create_index('tasks_project_id_idx', 'tasks', ['project_id'])
    op.create_index('tasks_project_status_idx', 'tasks', ['project_id', 'status'])
    # Partial index for active tasks only
    op.create_index(
        'tasks_due_date_active_idx',
        'tasks', ['due_date'],
        postgresql_where=sa.text('deleted_at IS NULL')
    )


def downgrade() -> None:
    op.drop_table('tasks')
    op.execute("DROP TYPE IF EXISTS task_status")

Row-Level Security (RLS) Policies

-- Enable RLS
ALTER TABLE tasks ENABLE ROW LEVEL SECURITY;
ALTER TABLE projects ENABLE ROW LEVEL SECURITY;

-- Create app role
CREATE ROLE app_user;

-- Users can only see tasks in their organization's projects
CREATE 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 records
CREATE POLICY tasks_no_deleted ON tasks
  FOR SELECT TO app_user
  USING (deleted_at IS NULL);

-- Only task creator or admin can delete
CREATE 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.ts
import { 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: 'admin@acme.com',
    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: faker.company.catchPhrase(),
    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