Advanced Customization — Flask-SQLAlchemy Documentation (3.1.x)

# 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] = mapped_column(String)

    class Employee(User):
        title: Mapped[str] = mapped_column(String)

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

    class TimestampModel(db.Model):
        __abstract__ = True
        created: Mapped[datetime] = mapped_column(DateTime, nullable=False, default=datetime.utcnow)
        updated: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)

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

    class Post(TimestampModel):
        id: Mapped[int] = mapped_column(Integer, primary_key=True)
        title: Mapped[str] = mapped_column(String, nullable=False)

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

    class TimestampMixin:
        created: Mapped[datetime] = mapped_column(DateTime, nullable=False, default=datetime.utcnow)
        updated: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)

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

## 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})