Advanced Customization

The various objects managed by the extension can be customized by passing arguments to the SQLAlchemy constructor.

Model Class

SQLAlchemy models all inherit from a declarative base class. This is exposed as db.Model in Flask-SQLAlchemy, which all models extend. This can be customized by subclassing the default and passing the custom class to model_class.

The following example gives every model an integer primary key, or a foreign key for joined-table inheritance.

Note

Integer primary keys for everything is not necessarily the best database design (that’s up to your project’s requirements), this is only an example.

from sqlalchemy import Integer, String, ForeignKey
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, declared_attr

class Base(DeclarativeBase):
    @declared_attr.cascading
    @classmethod
    def id(cls):
        for base in cls.__mro__[1:-1]:
            if getattr(base, "__table__", None) is not None:
                    return mapped_column(ForeignKey(base.id), primary_key=True)
            else:
                return mapped_column(Integer, primary_key=True)

db = SQLAlchemy(app, model_class=Base)

class User(db.Model):
    name: Mapped[str]

class Employee(User):
    title: Mapped[str]

Abstract Models and Mixins

If behavior is only needed on some models rather than all models, use an abstract model base class to customize only those models. For example, if some models should track when they are created or updated.

from datetime import datetime, timezone
from sqlalchemy.orm import Mapped, mapped_column

class TimestampModel(db.Model):
    __abstract__ = True
    created: Mapped[datetime] = mapped_column(default=lambda: datetime.now(timezone.utc))
    updated: Mapped[datetime] = mapped_column(default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))

class Author(db.Model):
    id: Mapped[int] = mapped_column(primary_key=True)
    username: Mapped[str] = mapped_column(unique=True)

class Post(TimestampModel):
    id: Mapped[int] = mapped_column(primary_key=True)
    title: Mapped[str]

This can also be done with a mixin class, inheriting from db.Model separately.

class TimestampMixin:
    created: Mapped[datetime] = mapped_column(default=lambda: datetime.now(timezone.utc))
    updated: Mapped[datetime] = mapped_column(default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))

class Post(TimestampMixin, db.Model):
    id: Mapped[int] = mapped_column(primary_key=True)
    title: Mapped[str]

Disabling Table Name Generation

Some projects prefer to set each model’s __tablename__ manually rather than relying on Flask-SQLAlchemy’s detection and generation. The simple way to achieve that is to set each __tablename__ and not modify the base class. However, the table name generation can be disabled by setting disable_autonaming=True in the SQLAlchemy constructor.

class Base(sa_orm.DeclarativeBase):
    pass

db = SQLAlchemy(app, model_class=Base, disable_autonaming=True)

Session Class

Flask-SQLAlchemy’s Session class chooses which engine to query based on the bind key associated with the model or table. However, there are other strategies such as horizontal sharding that can be implemented with a different session class. The class_ key to the session_options argument to the extension to change the session class.

Flask-SQLAlchemy will always pass the extension instance as the db argument to the session, so it must accept that to continue working. That can be used to get access to db.engines.

from sqlalchemy.ext.horizontal_shard import ShardedSession
from flask_sqlalchemy.session import Session

class CustomSession(ShardedSession, Session):
    ...

db = SQLAlchemy(session_options={"class_": CustomSession})

Query Class

Warning

The query interface is considered legacy in SQLAlchemy. This includes session.query, Model.query, db.Query, and lazy="dynamic" relationships. Prefer using session.execute(select(...)) instead.

It is possible to customize the query interface used by the session, models, and relationships. This can be used to add extra query methods. For example, you could add a get_or method that gets a row or returns a default.

from flask_sqlalchemy.query import Query

class GetOrQuery(Query):
    def get_or(self, ident, default=None):
        out = self.get(ident)

        if out is None:
            return default

        return out

db = SQLAlchemy(query_class=GetOrQuery)

user = User.query.get_or(user_id, anonymous_user)

Passing the query_class argument will customize db.Query, db.session.query, Model.query, and db.relationship(lazy="dynamic") relationships. It’s also possible to customize these on a per-object basis.

To customize a specific model’s query property, set the query_class attribute on the model class.

class User(db.Model):
    query_class = GetOrQuery

To customize a specific dynamic relationship, pass the query_class argument to the relationship.

db.relationship(User, lazy="dynamic", query_class=GetOrQuery)

To customize only session.query, pass the query_cls key to the session_options argument to the constructor.

db = SQLAlchemy(session_options={"query_cls": GetOrQuery})