Applying Global Filters to All Tables in SQLAlchemy Queries

• You want all queries, regardless of the table, to have a specific filtering condition applied.

Approaches:

1. Filter on Demand:

2. Event Listeners:

Things to Consider:

• Relationships: If your filter needs to apply to related objects as well (e.g., filtering users with active profiles), the before_compile approach might need adjustments.
• Performance: Adding a global filter might affect query performance slightly.

Recommendation:

The "filter on demand" approach is generally simpler to implement for most cases. If you need more control or the filter affects relationships, explore the before_compile event listener approach with caution.

from sqlalchemy import create_engine, Column, Integer, String, select

engine = create_engine("sqlite:///mydatabase.db")

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    is_active = Column(Integer)

    def __repr__(self):
        return f"User(id={self.id}, name='{self.name}', is_active={self.is_active})"

# Sample data (assuming is_active=1 for active users)
Base.metadata.create_all(engine)
session = Session(engine)
session.add_all([User(name="Alice", is_active=1), User(name="Bob", is_active=0), User(name="Charlie", is_active=1)])
session.commit()

# Base query (without filter)
query = select(User)

# Apply filter during iteration
active_users = [user for user in query if user.is_active == 1]

for user in active_users:
    print(user)

session.close()

This code defines a User class and adds sample data. The query retrieves all users, but the active users are printed by applying the filter during iteration (user.is_active == 1).

Event Listener (before_compile):

from sqlalchemy import create_engine, Column, Integer, String, select, event
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

engine = create_engine("sqlite:///mydatabase.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String)
    is_active = Column(Integer)

    def __repr__(self):
        return f"User(id={self.id}, name='{self.name}', is_active={self.is_active})"

# Sample data (assuming is_active=1 for active users)
Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()
session.add_all([User(name="Alice", is_active=1), User(name="Bob", is_active=0), User(name="Charlie", is_active=1)])
session.commit()


def apply_active_filter(query):
    if isinstance(query, select):
        query = query.where(User.is_active == 1)
    return query

event.listen(select, 'before_compile', apply_active_filter)

# Base query (automatically filtered)
active_users = session.query(User).all()

for user in active_users:
    print(user)

session.close()

This code uses an event listener to add a where clause filtering for active users (User.is_active == 1) before any select query is compiled. This affects all queries automatically but requires attaching the listener at the beginning.

• Create a custom query class inheriting from sqlalchemy.orm.query.Query.
• Override the filter method in your class to apply the global filter along with any user-provided filters.
• Use your custom query class throughout the application for all database interactions.

This approach keeps the queries clean but requires modifying how you interact with the database.

Mixins:

• Define a mixin class containing the global filter logic as a method (e.g., apply_global_filter).
• Inherit this mixin class in your main query classes.
• Call the apply_global_filter method within your queries to incorporate the global filter.

This approach offers reusability but might clutter individual query definitions.

Context-Aware Filters:

• Implement a context manager or a decorator that sets up the global filter for the duration of its execution.
• Wrap your database access logic within this context to ensure the filter is applied.

This approach can be useful for temporary filtering needs but might require careful context management.

Choosing the Right Method:

The best method depends on your specific scenario. Here's a guideline:

• For simple filtering needs, "filter on demand" is easy to implement.
• If you need reusability and your application uses many queries, a custom query class or mixins might be better.
• For temporary filtering within specific contexts, a context manager or decorator could be suitable.