In SQLAlchemy, composite fields allow us to group multiple columns together as a single unit. This can be useful in scenarios where we want to encapsulate related information and work with it as a single entity. So, instead of dealing with individual columns, we can treat them as a composite field.

Defining a Composite Field

To define a composite field in SQLAlchemy, we need to create a custom class that inherits from the sqlalchemy.types.TypeDecorator class. This class provides a framework for creating custom types in SQLAlchemy.

from sqlalchemy import types

class CompositeField(types.TypeDecorator):
    impl = types.String
    
    def __init__(self, field1, field2):
        self.field1 = field1
        self.field2 = field2
    
    def process_bind_param(self, value, dialect):
        if value is not None:
            return f'{value.field1}|{value.field2}'
    
    def process_result_value(self, value, dialect):
        if value is not None:
            field1, field2 = value.split('|')
            return MyCompositeField(field1=field1, field2=field2)
    
    def coerce_compared_value(self, op, value):
        return self
        

In this example, we’ve created a CompositeField class that combines two fields (field1 and field2) into a single composite field. The process_bind_param method is responsible for converting the composite field into a string representation that can be stored in the database. Similarly, the process_result_value method converts the stored value back into a composite field object.

Using a Composite Field

Once we have defined a composite field, we can use it in our SQLAlchemy models. Let’s say we have a User model with a composite field address that consists of a street and city field.

from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    
    id = Column(Integer, primary_key=True)
    name = Column(String)
    address = Column(CompositeField('street', 'city'))
    
    def __repr__(self):
        return f"<User(name='{self.name}', address='{self.address}')>"

In this example, the address column is declared as a composite field using the CompositeField class we defined earlier. Now, whenever we create a new User object, we can assign a composite field value to the address attribute, and SQLAlchemy will automatically handle the conversion and storage of the composite field in the database.

Querying Composite Fields

To query composite fields in SQLAlchemy, we can use the column attribute of the composite field. For example, to query all users with a specific city in their address, we can use the following code:

from sqlalchemy.orm import sessionmaker

Session = sessionmaker(bind=engine)
session = Session()

users = session.query(User).filter(User.address.column.field2 == 'New York').all()

In this example, we filter the User objects based on the field2 attribute of the address composite field, which represents the city.

Conclusion

SQLAlchemy composite fields provide a convenient way to work with related data as a single entity. By defining custom composite field classes, we can encapsulate multiple columns into a single field and utilize them in our SQLAlchemy models. This allows for more organized and expressive code when dealing with complex data structures.