SQLAlchemy Patterns for Flask (sync delta)

Load python-foundation:sqlalchemy-patterns via the Skill tool FIRST. It contains the shared SQLAlchemy 2.0 core: detection, Mapped/mapped_column model definition, column type guidance, select() querying, relationship structure, lazy-strategy overview, and migration metadata rules. This skill covers only the Flask-SQLAlchemy delta.

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Version detection

Beyond the foundation skill's SQLAlchemy detection, determine the Flask-SQLAlchemy version:

grep -E "flask.sqlalchemy" requirements.txt pyproject.toml

• Flask-SQLAlchemy 3.x (released 2022+): Uses SQLAlchemy 2.0 under the hood. Supports Mapped/mapped_column style with db.Model. This is the baseline for new projects.
• Flask-SQLAlchemy 2.x (legacy): Uses db.Column() style. Still common in existing projects.

Always use Flask-SQLAlchemy 3.x style for new code. When working in an existing project using 2.x style throughout, match the existing style. Never mix db.Column() and mapped_column() in the same model.

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Extension setup

Define extensions at module level in app/extensions.py and call .init_app(app) in the factory. This avoids circular imports and allows multiple app instances for testing.

# app/extensions.py
from flask_migrate import Migrate
from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()
migrate = Migrate()

Initialize in the factory:

# app/__init__.py
from app.extensions import db, migrate


def create_app(config_name: str = "development") -> Flask:
    app = Flask(__name__)
    app.config.from_object(config_by_name[config_name])

    db.init_app(app)
    migrate.init_app(app, db)

    return app

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Model definition with db.Model

Models inherit from db.Model instead of a hand-written DeclarativeBase; all mapped-column, typing, and relationship rules from the foundation skill apply unchanged.

from datetime import datetime

from sqlalchemy import DateTime, String, func
from sqlalchemy.orm import Mapped, mapped_column, relationship

from app.extensions import db


class User(db.Model):
    __tablename__ = "users"

    id: Mapped[int] = mapped_column(primary_key=True)
    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
    created_at: Mapped[datetime] = mapped_column(
        DateTime(timezone=True), server_default=func.now()
    )

    orders: Mapped[list["Order"]] = relationship(
        "Order", back_populates="user", lazy="select"
    )

Flask-SQLAlchemy 2.x with db.Column (legacy)

from app.extensions import db


class User(db.Model):
    __tablename__ = "users"

    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(255), unique=True, nullable=False)
    display_name = db.Column(db.String(100), nullable=False)
    hashed_password = db.Column(db.String(255), nullable=False)
    is_active = db.Column(db.Boolean, default=True)
    created_at = db.Column(db.DateTime(timezone=True), server_default=db.func.now())

    orders = db.relationship("Order", back_populates="user", lazy="select")

    def __repr__(self) -> str:
        return f"<User id={self.id} email={self.email!r}>"

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Session lifecycle and querying (synchronous)

Flask-SQLAlchemy uses synchronous sessions. All queries are blocking — no await, no async generators. Statement construction follows the foundation skill; execution goes through db.session.

from sqlalchemy import select

from app.extensions import db
from app.users.models import User


def get_user_by_id(user_id: int) -> User | None:
    return db.session.get(User, user_id)


def get_user_by_email(email: str) -> User | None:
    return db.session.execute(
        select(User).where(User.email == email)
    ).scalar_one_or_none()


def create_user(email: str, hashed_password: str, display_name: str) -> User:
    user = User(email=email, hashed_password=hashed_password, display_name=display_name)
    db.session.add(user)
    db.session.flush()  # assigns id without committing
    return user


def delete_user(user: User) -> None:
    db.session.delete(user)

Session lifecycle in Flask: Flask-SQLAlchemy automatically calls db.session.remove() at the end of each request via teardown_appcontext. This closes the session and returns the connection to the pool. You do not need to call db.session.close() manually in view functions.

When to commit: call db.session.commit() in the view function or service after all writes are complete for the request. Use db.session.flush() inside a unit of work to get the generated PK without committing.

@orders_bp.route("/", methods=["POST"])
@login_required
def create_order_view():
    data = order_create_schema.load(request.get_json())
    order = create_order(data)
    db.session.commit()
    return jsonify(order_schema.dump(order)), 201

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Lazy loading in synchronous Flask

The foundation skill's lazy-strategy catalog applies; in synchronous Flask the sync-only strategies are also valid:

• lazy="select" — the SQLAlchemy default; loads on first attribute access with a separate SELECT. Safe in synchronous Flask (unlike async FastAPI, where it can block the event loop).
• lazy="joined" — JOIN in the same query; best for one-to-one relations or small, always-needed collections.
• lazy="subquery" — valid in synchronous Flask (unlike async where it is not supported). Useful for loading collections alongside the parent.
• lazy="dynamic" — deprecated in 2.0, do not use (see foundation skill).

For N+1 prevention on list endpoints, use query-time options(joinedload(...)) or options(selectinload(...)) as shown in the foundation skill.

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Flask-Migrate integration

Flask-Migrate wraps Alembic. Running flask db init scaffolds the migrations/ directory including migrations/env.py and migrations/alembic.ini. Do not hand-write env.py — Flask-Migrate generates a synchronous configuration automatically.

migrations/
    alembic.ini
    env.py          — generated by flask db init; imports db.metadata
    script.py.mako
    versions/       — generated migration scripts live here

Per the foundation skill's metadata rules, all model modules must be imported before target_metadata = db.metadata. The Flask convention is to import them in app/models/__init__.py or in the app factory:

# app/__init__.py
def create_app(config_name: str = "development") -> Flask:
    app = Flask(__name__)
    app.config.from_object(config_by_name[config_name])

    db.init_app(app)
    migrate.init_app(app, db)

    # Import models so Flask-Migrate sees them in db.metadata
    with app.app_context():
        from app.users import models as _user_models  # noqa: F401
        from app.orders import models as _order_models  # noqa: F401

    _register_blueprints(app)
    return app

Flask-migrate-specialist runs flask db migrate and flask db upgrade. Flask-architect only defines the models. Never call flask db commands from flask-architect.

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Flask-specific anti-patterns

Anti-pattern	Problem	Correct approach
db.session.commit() in every helper function	Makes unit testing harder; scattered transaction boundaries	Commit at the end of the request in the view or service layer
Mixing db.Column() and mapped_column()	Produces inconsistent metadata; confuses tooling	One style per project; prefer mapped_column() for new code
Importing models only in routers	Flask-Migrate's env.py never sees them; --autogenerate misses tables	Import all models in the app factory or app/models/__init__.py