How to Set an Array Column with an Empty Array as Default in SQLAlchemy + PostgreSQL
Storing lists or collections within a database table often requires using array columns. In PostgreSQL, arrays are a powerful feature for representing ordered sets of data. However, when using SQLAlchemy to define your database schema, you might encounter challenges when setting an empty array as the default value for a column. This article will guide you through the process and explain the necessary steps to achieve this.
The Problem
You want to define a table with a column of type ARRAY in PostgreSQL. This column should hold lists of values, and you desire an empty array to be the default value for newly inserted rows.
Scenario and Original Code
Let's consider a simple example. We want to create a table named products with a column named tags of type ARRAY that holds a list of string values.
from sqlalchemy import Column, Integer, String, ARRAY
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class Product(Base):
__tablename__ = 'products'
id = Column(Integer, primary_key=True)
name = Column(String)
tags = Column(ARRAY(String))
engine = create_engine('postgresql://user:password@host:port/database')
Session = sessionmaker(bind=engine)
session = Session()
Base.metadata.create_all(engine)
The above code defines the Product model and initializes the database connection. However, it does not specify a default value for the tags column. This results in an empty array being returned when retrieving the tags field from a new Product object.
Solution
To set an empty array as the default value for the tags column, we need to use the server_default argument within the Column definition.
from sqlalchemy import Column, Integer, String, ARRAY
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class Product(Base):
__tablename__ = 'products'
id = Column(Integer, primary_key=True)
name = Column(String)
tags = Column(ARRAY(String), server_default='{}')
engine = create_engine('postgresql://user:password@host:port/database')
Session = sessionmaker(bind=engine)
session = Session()
Base.metadata.create_all(engine)
In this modified code, the server_default argument is set to '{}'. This instructs SQLAlchemy to create the table with a default value of an empty array for the tags column.
Explanation
The server_default argument allows us to specify a default value for a column that will be applied by the database server (PostgreSQL in this case).
- The
'{}'represents an empty array in the PostgreSQL syntax. - When creating a new row in the
productstable, thetagscolumn will automatically be populated with an empty array.
Additional Considerations
- Data Types: Be sure to use the correct data type within the
ARRAYconstructor to match the elements of your arrays (e.g.,ARRAY(String)for a list of strings). - Database Version: This solution works with recent versions of PostgreSQL. Older versions might require slightly different syntax for defining default array values.
Conclusion
By utilizing the server_default argument in SQLAlchemy, you can effortlessly define empty arrays as default values for your array columns in PostgreSQL. This practice ensures consistency and streamlines data management by eliminating the need to manually initialize empty arrays for new entries.
