In SQLAlchemy, custom query classes allow you to define reusable query methods or attributes for your models. This can help simplify your code and make it easier to manage complex queries. In this blog post, we’ll explore how to create and use custom query classes in SQLAlchemy.

What are Custom Query Classes?

Custom query classes in SQLAlchemy are Python classes that inherit from the Query class provided by SQLAlchemy. They allow you to define custom methods or attributes that can be used for querying the database. This can be useful when you want to encapsulate complex queries, add additional filters, or provide convenience methods for frequently used queries.

Creating a Custom Query Class

To create a custom query class, you need to define a class that inherits from the Query class. Let’s say we have a User model and we want to create a custom query class for it:

from sqlalchemy import Column, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Query

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'

    id = Column(String, primary_key=True)
    name = Column(String)

class CustomUserQuery(Query):
    def active_users(self):
        return self.filter(User.active == True)

    def order_by_name(self):
        return self.order_by(User.name)

In the above example, we define a CustomUserQuery class that inherits from Query. Inside this class, we define two methods: active_users and order_by_name. The active_users method filters the query to only return active users, while order_by_name orders the result by the user’s name.

Using Custom Query Classes

To use the custom query class, we need to associate it with the User model using the query_class attribute. Here’s an example:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

# Create engine and session
engine = create_engine('sqlite:///mydatabase.db')
Session = sessionmaker(bind=engine)
session = Session()

# Associate custom query class with User model
User.query = session.query_property(CustomUserQuery)

# Use the custom query methods
active_users = User.query.active_users().all()
users_ordered_by_name = User.query.order_by_name().all()

In the above code, we create a SQLAlchemy session and associate our CustomUserQuery class with the User model using session.query_property(). We can then use the custom query methods active_users() and order_by_name() to perform the desired queries.

Conclusion

By creating custom query classes in SQLAlchemy, you can enhance the flexibility and reusability of your database queries. Custom query classes allow you to encapsulate complex queries, add additional filters, or provide convenience methods for frequently used queries. Incorporating custom query classes into your SQLAlchemy workflow can help streamline your code and improve its readability.