PostgreSQL Schema Design (SQLModel)

Quick Start

python

import uuid
from datetime import datetime
from sqlmodel import Field, Relationship, SQLModel

class TimestampMixin(SQLModel):
    created_at: datetime = Field(default_factory=datetime.utcnow)
    updated_at: datetime = Field(default_factory=datetime.utcnow)

class User(TimestampMixin, table=True):
    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
    email: str = Field(unique=True, index=True)
    name: str = Field(index=True)

class Post(TimestampMixin, table=True):
    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
    title: str = Field(index=True)
    author_id: uuid.UUID = Field(foreign_key="user.id", index=True)
    author: User | None = Relationship(back_populates="posts")

Decision Guide

Primary Key: UUID vs Integer

Use UUID	Use Integer
Distributed systems	Single database
Hide record count	Need ordering by ID
Client-generated IDs	Simpler debugging
API exposure	Smaller storage

python

# UUID (recommended default)
id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)

# Integer auto-increment
id: int | None = Field(default=None, primary_key=True)

Table Names: Singular

SQLModel converts UserAccount → useraccount. Override if needed:

python

class UserAccount(SQLModel, table=True):
    __tablename__ = "user_account"

Always Index Foreign Keys

PostgreSQL does NOT auto-index FKs:

python

# WRONG - no index on FK
author_id: uuid.UUID = Field(foreign_key="user.id")

# CORRECT - indexed FK
author_id: uuid.UUID = Field(foreign_key="user.id", index=True)

Relationship Patterns

One-to-Many

python

class Team(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    members: list["User"] = Relationship(back_populates="team")

class User(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    team_id: int | None = Field(default=None, foreign_key="team.id", index=True)
    team: Team | None = Relationship(back_populates="members")

Many-to-Many

python

class UserRoleLink(SQLModel, table=True):
    user_id: uuid.UUID = Field(foreign_key="user.id", primary_key=True)
    role_id: uuid.UUID = Field(foreign_key="role.id", primary_key=True)

class User(SQLModel, table=True):
    roles: list["Role"] = Relationship(back_populates="users", link_model=UserRoleLink)

class Role(SQLModel, table=True):
    users: list[User] = Relationship(back_populates="roles", link_model=UserRoleLink)

Self-Referential (Tree)

python

class Category(SQLModel, table=True):
    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
    parent_id: uuid.UUID | None = Field(default=None, foreign_key="category.id", index=True)
    parent: "Category | None" = Relationship(
        back_populates="children",
        sa_relationship_kwargs={"remote_side": "Category.id"}
    )
    children: list["Category"] = Relationship(back_populates="parent")

PostgreSQL-Specific Features

JSONB Column

python

from sqlalchemy import Column
from sqlalchemy.dialects.postgresql import JSONB

class User(SQLModel, table=True):
    metadata: dict = Field(default={}, sa_column=Column(JSONB))

Composite/Partial Index

python

from sqlalchemy import Index, text

class User(SQLModel, table=True):
    __table_args__ = (
        Index('idx_user_tenant_email', 'tenant_id', 'email', unique=True),
        Index('idx_user_active', 'id', postgresql_where=text('deleted_at IS NULL')),
    )

Check Constraint

python

class Order(SQLModel, table=True):
    __table_args__ = (
        CheckConstraint("status IN ('pending', 'shipped', 'delivered')"),
    )

Resources

Need	Reference
ERD creation	assets/erd-template.md
Migration SQL	assets/migration-template.sql
Pre-deploy checklist	assets/schema-checklist.md
Naming rules	references/naming-conventions.md
Normalization (1NF-3NF)	references/normalization.md
Index strategies	references/index-guidelines.md
Common patterns (RBAC, audit, soft-delete, multi-tenant)	references/common-patterns.md

Scripts

Generate SQLModel boilerplate:

bash

python scripts/generate_models.py --example --output models.py

Checklist (Critical Items)

• Every table has a primary key
• All foreign keys are indexed
• created_at/updated_at on mutable tables
• Unique constraints where business rules require
• No sensitive data in plain text
• Soft delete uses partial index for active records