Filtering Magic: Adding Automatic Conditions to SQLAlchemy Relations

• Soft deletion: Instead of actually deleting records, mark them as "deleted" and filter them out by default.
• Filtering active users: Only retrieve users whose status is "active" by default.

There are several ways to achieve this:

Using primaryjoin argument in the relation definition:

This approach modifies the JOIN clause in the query by adding a filter condition.

from sqlalchemy import Column, Integer, Boolean, ForeignKey
from sqlalchemy.orm import relationship

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(String(80), unique=True)
    is_active = Column(Boolean, default=True)

    posts = relationship("Post", backref='user', primaryjoin="user.id == Post.user_id and Post.is_deleted == False")

class Post(Base):
    __tablename__ = 'posts'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('users.id'))
    content = Column(Text)
    is_deleted = Column(Boolean, default=False)

Here, the posts relationship in the User model uses the primaryjoin argument to filter out deleted posts (where is_deleted is False). Any query on User will automatically apply this filter on the joined posts relation.

Using a custom query class:

This method involves creating a custom query class inheriting from sqlalchemy.orm.query.Query and overriding its behavior.

from sqlalchemy import orm

class UserQuery(orm.Query):
    def filter_by_active(self):
        return self.filter_by(is_active=True)

class User(Base):
    __tablename__ = 'users'
    # ... (same as previous example)

    query = orm.sessionmaker(bind=engine).query_class(UserQuery)

Here, the UserQuery class overrides the filter_by_active method to automatically filter by active users whenever the query object is used.

Using event listeners:

This approach uses SQLAlchemy events to intercept query construction and apply filters dynamically.

from sqlalchemy import event

def filter_inactive_users(mapper, connection, instance):
    if isinstance(instance, UserQuery):
        instance.filter_by(is_active=True)

event.listen(UserQuery, 'before_query', filter_inactive_users)

This code defines an event listener function that checks if the query is of type UserQuery and then applies the is_active filter.

Related Issues and Solutions:

• Performance: Applying filters in the primaryjoin can affect performance for large datasets as it adds an extra condition to the JOIN operation. Consider using eager loading (e.g., options(orm.joinedload('posts'))) to pre-load related objects if needed.
• Dynamic conditions: While the above examples show static filters, you can construct dynamic filters based on user input or other criteria.

Choosing the best approach depends on your specific needs and the complexity of your filtering conditions.