Peewee is a simple yet powerful Object Relational Mapping (ORM) library for Python. It provides a convenient way to interact with databases by abstracting the underlying database-specific details.

When working with databases, it is common to have default values for columns or enforce constraints on the data being inserted. In this blog post, we will explore how to set default values and constraints in Peewee.

Setting Default Values

Default values are used to provide a fallback value for a column when no value is explicitly specified during an insert operation. Peewee allows you to set default values for columns using the default parameter in the field definition.

Let’s say we have a User model with a status column that defaults to 'active':

from peewee import Model, CharField, TextField

class User(Model):
    username = CharField()
    status = CharField(default='active')
    bio = TextField()

    class Meta:
        database = db

In the above example, if we insert a new user without specifying the status column, it will automatically be set to 'active'. However, if we explicitly set a value for the status column during the insert, the specified value will be used instead.

Constraints

Constraints are rules applied to the database to ensure the integrity of the data being inserted or updated. Peewee supports various constraints such as NOT NULL, UNIQUE, PRIMARY KEY, CHECK, and FOREIGN KEY.

NOT NULL Constraint

The NOT NULL constraint ensures that a column cannot have a NULL value. In Peewee, you can specify the null parameter in the field definition to enforce the NOT NULL constraint:

from peewee import Model, CharField

class User(Model):
    username = CharField(unique=True)
    password = CharField(null=False)

    class Meta:
        database = db

In the above example, the username column is unique and the password column must not be NULL during insert or update operations.

UNIQUE Constraint

The UNIQUE constraint ensures that the values in a column are unique. Peewee provides the unique parameter to set the UNIQUE constraint on a column:

from peewee import Model, CharField

class User(Model):
    email = CharField(unique=True)
    username = CharField(unique=True)

    class Meta:
        database = db

In the above example, both the email and username columns have the UNIQUE constraint.

CHECK Constraint

The CHECK constraint allows you to specify a condition that must be true for the data being inserted or updated. Peewee supports the constraints parameter to add a CHECK constraint:

from peewee import Model, CharField

class User(Model):
    username = CharField()
    age = IntegerField(constraints=[Check('age > 0')])

    class Meta:
        database = db

In the above example, the age column has a CHECK constraint that ensures the age is greater than 0.

Conclusion

In this blog post, we explored how to set default values and constraints in Peewee. Setting default values and constraints are essential for maintaining data integrity and enforcing business rules. With Peewee, you can easily define default values and constraints for your database columns, making it easier to work with data in your applications.