SQLModel Patterns

Problem Statement

SQLModel combines Pydantic and SQLAlchemy, blurring the line between models and schemas. Async SQLAlchemy has different rules than sync. Mistakes here cause data corruption, N+1 queries, and hard-to-debug errors.

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Pattern: Eager Loading for Async

Problem: Lazy loading doesn't work with async SQLAlchemy. Accessing relationships without eager loading raises errors.

# ❌ WRONG: Lazy loading fails in async
result = await session.execute(select(User).where(User.id == user_id))
user = result.scalar_one()
assessments = user.assessments  # ERROR: greenlet_spawn has not been called

# ✅ CORRECT: selectinload for collections
from sqlalchemy.orm import selectinload

result = await session.execute(
    select(User)
    .where(User.id == user_id)
    .options(selectinload(User.assessments))
)
user = result.scalar_one()
assessments = user.assessments  # Works - already loaded

# ✅ CORRECT: joinedload for single relationships
from sqlalchemy.orm import joinedload

result = await session.execute(
    select(Assessment)
    .where(Assessment.id == assessment_id)
    .options(joinedload(Assessment.user))
)
assessment = result.scalar_one()
user = assessment.user  # Works - already loaded

When to use which:

Relationship	Loading Strategy
One-to-many (collections)	selectinload()
Many-to-one (single)	joinedload()
Nested relationships	Chain: .options(selectinload(A.b).selectinload(B.c))

---

Pattern: N+1 Query Detection

Problem: Fetching related objects one-by-one instead of in batch.

# ❌ WRONG: N+1 queries
users = await session.execute(select(User))
for user in users.scalars():
    # Each access triggers a query!
    print(user.team.name)  # Query 1, 2, 3... N

# ✅ CORRECT: Single query with eager loading
users = await session.execute(
    select(User).options(joinedload(User.team))
)
for user in users.scalars():
    print(user.team.name)  # No additional queries

# Detection: Enable SQL echo in development
engine = create_async_engine(DATABASE_URL, echo=True)
# Watch logs for repeated similar queries

---

Pattern: Model vs Schema Separation

Problem: SQLModel blurs models (DB) and schemas (API). Need clear separation.

# Database Model - represents table
class User(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)
    email: str = Field(index=True, unique=True)
    hashed_password: str  # Never expose this
    created_at: datetime = Field(default_factory=datetime.utcnow)

    # Relationships
    assessments: list["Assessment"] = Relationship(back_populates="user")

# API Schema - Create (input)
class UserCreate(SQLModel):
    email: str
    password: str  # Plain password, will be hashed

# API Schema - Read (output)
class UserRead(SQLModel):
    id: UUID
    email: str
    created_at: datetime
    # Note: No password field!

# API Schema - Update (partial)
class UserUpdate(SQLModel):
    email: str | None = None
    password: str | None = None

Naming convention:

• ModelName - Database table model
• ModelNameCreate - Input for creation
• ModelNameRead - Output for reading
• ModelNameUpdate - Input for partial updates

---

Pattern: Session State Management

Problem: Understanding expire_on_commit and when objects become stale.

# This codebase setting
async_session = async_sessionmaker(
    engine,
    expire_on_commit=False,  # Objects stay valid after commit
)

# With expire_on_commit=False:
user = User(email="test@example.com")
session.add(user)
await session.commit()
print(user.email)  # Works - object still valid

# With expire_on_commit=True (default):
await session.commit()
print(user.email)  # Would need refresh() first

# ✅ CORRECT: Refresh when you need DB-generated values
await session.commit()
await session.refresh(user)  # Get id, created_at, updated DB values
return user

---

Pattern: UUID Handling

Problem: Inconsistent UUID handling between Python and PostgreSQL.

from uuid import UUID, uuid4

# ✅ CORRECT: UUID with default factory
class Assessment(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)
    user_id: UUID = Field(foreign_key="user.id")

# ✅ CORRECT: UUID in queries
await session.execute(
    select(Assessment).where(Assessment.id == UUID("..."))
)

# ❌ WRONG: String comparison
await session.execute(
    select(Assessment).where(Assessment.id == "some-uuid-string")
)

# ✅ CORRECT: Converting in API layer
@router.get("/assessments/{assessment_id}")
async def get_assessment(assessment_id: UUID):  # FastAPI converts string to UUID
    ...

---

Pattern: Nullable Fields

Problem: SQLModel requires specific syntax for optional fields.

# ✅ CORRECT: Optional field with None default
class Assessment(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)
    title: str  # Required
    description: str | None = Field(default=None)  # Optional
    completed_at: datetime | None = Field(default=None)  # Optional

    # Foreign key that's optional
    coach_id: UUID | None = Field(default=None, foreign_key="user.id")

# ❌ WRONG: Optional without Field default
class BadModel(SQLModel, table=True):
    description: str | None  # Missing default - causes issues

---

Pattern: Relationship Definitions

from sqlmodel import Relationship

class User(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)

    # One-to-many: User has many assessments
    assessments: list["Assessment"] = Relationship(back_populates="user")

    # One-to-many: User has many answers
    answers: list["UserAnswer"] = Relationship(back_populates="user")

class Assessment(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)
    user_id: UUID = Field(foreign_key="user.id")

    # Many-to-one: Assessment belongs to user
    user: User = Relationship(back_populates="assessments")

    # One-to-many: Assessment has many questions
    questions: list["Question"] = Relationship(back_populates="assessment")

class Question(SQLModel, table=True):
    id: UUID = Field(default_factory=uuid4, primary_key=True)
    assessment_id: UUID = Field(foreign_key="assessment.id")

    # Many-to-one
    assessment: Assessment = Relationship(back_populates="questions")

---

Pattern: Query Patterns

# Get one or None
result = await session.execute(
    select(User).where(User.id == user_id)
)
user = result.scalar_one_or_none()

# Get one or raise
user = result.scalar_one()  # Raises if 0 or >1 results

# Get list
result = await session.execute(
    select(Assessment).where(Assessment.user_id == user_id)
)
assessments = result.scalars().all()

# Get with pagination
result = await session.execute(
    select(Assessment)
    .where(Assessment.user_id == user_id)
    .order_by(Assessment.created_at.desc())
    .offset(skip)
    .limit(limit)
)

# Count
result = await session.execute(
    select(func.count()).select_from(Assessment).where(...)
)
count = result.scalar_one()

# Exists check
result = await session.execute(
    select(exists().where(User.email == email))
)
email_exists = result.scalar()

---

Pattern: Upsert (Insert or Update)

from sqlalchemy.dialects.postgresql import insert

# ✅ CORRECT: PostgreSQL upsert
stmt = insert(UserAnswer).values(
    user_id=user_id,
    question_id=question_id,
    value=value,
)
stmt = stmt.on_conflict_do_update(
    index_elements=["user_id", "question_id"],
    set_={"value": value, "updated_at": datetime.utcnow()},
)
await session.execute(stmt)
await session.commit()

---

References

• SQLModel documentation: https://sqlmodel.tiangolo.com/
• SQLAlchemy 2.0 documentation: https://docs.sqlalchemy.org/

---

Common Issues

Issue	Likely Cause	Solution
"greenlet_spawn has not been called"	Lazy loading in async	Use selectinload/joinedload
N+1 queries (slow)	Missing eager loading	Add appropriate loading strategy
"Object not bound to session"	Using object after session closed	Keep operations within session scope
Stale data	Missing refresh()	Call refresh() after commit
"None is not valid" for UUID	Missing default_factory	Add Field(default_factory=uuid4)

---

Detection Commands

# Find lazy relationship access
grep -rn "\.scalars\(\)" --include="*.py" -A5 | grep -E "\.\w+\s*$"

# Find models missing relationship loading
grep -rn "select(" --include="*.py" | grep -v "options("

# Check for N+1 in logs (with echo=True)
# Look for repeated similar queries