Enforcing Data Integrity: Validation and Modification Techniques for Numbers in SQLAlchemy

2024-07-27

SQLAlchemy offers a built-in decorator called @validates to define validation logic for model attributes. You can use this to intercept numbers before they are inserted or updated in the database.

Here's an example:

from sqlalchemy import Column, Integer, Float

class MyModel(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    value = Column(Float, nullable=False)

    @validates('value')
    def validate_value(self, attr, value):
        if value < 0:
            raise ValueError("Value cannot be negative")
        # You can modify the value here before insert/update
        value *= 10  # Multiply by 10 (example modification)
        return value

In this example, the validate_value function ensures the value is non-negative and optionally modifies it by multiplying by 10 before it's inserted or updated.

Event Listeners:

SQLAlchemy allows attaching event listeners to specific operations like before_insert and before_update. These listeners are functions that get called before the actual database operation happens.

from sqlalchemy import event

def modify_numbers(mapper, connection, instance):
    # Modify numbers in instance here (example: add 5 to all)
    for attr in instance.__dict__:
        if isinstance(instance.__dict__[attr], (int, float)):
            instance.__dict__[attr] += 5

event.listen(MyModel, 'before_insert', modify_numbers)
event.listen(MyModel, 'before_update', modify_numbers)

This code defines a function modify_numbers that iterates through the attributes of the model instance and adds 5 to any number it finds (integer or float). This function is then attached as a listener for both before_insert and before_update events on the MyModel class.

Choosing the Right Approach:

Use event listeners for broader modifications across multiple attributes or for generic logic before insert/update.
Use @validates for specific attribute validation and potential modification logic.

from sqlalchemy import Column, Integer, Float

class MyModel(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    value = Column(Float, nullable=False)

    @validates('value')
    def validate_value(self, attr, value):
        if isinstance(value, str):
            value = value.replace(",", "")  # Remove commas if present
        value = float(value)
        if value < 0:
            raise ValueError("Value cannot be negative")
        return value

Example 2: Event Listeners

This example modifies all numbers (integers and floats) in the model instance by multiplying them by 2 before insert/update.

from sqlalchemy import event

class MyModel(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    value1 = Column(Integer)
    value2 = Column(Float)

def modify_numbers(mapper, connection, instance):
    for attr, value in instance.__dict__.items():
        if isinstance(value, (int, float)):
            setattr(instance, attr, value * 2)  # Modify value directly

event.listen(MyModel, 'before_insert', modify_numbers)
event.listen(MyModel, 'before_update', modify_numbers)

You can define a custom data type that inherits from existing numeric types like Integer or Float and override methods like process_bind_param to perform modifications before data is sent to the database.

This approach offers more granular control but requires creating a new class.

Property Decorators:

SQLAlchemy allows defining properties using decorators like @property. You can define a getter and potentially a setter for the property. The getter can perform any necessary modifications before returning the value for database operations.

This method keeps your model definition cleaner but might be less flexible for complex modifications.

Custom INSERT/UPDATE statements:

For more control, you can construct custom INSERT or UPDATE statements using SQLAlchemy's core API. These statements allow embedding logic to modify values directly before they are inserted or updated.

This approach offers maximum flexibility but requires writing more complex SQL code.

Use custom INSERT/UPDATE statements for situations requiring intricate logic or bypassing SQLAlchemy's higher-level functionalities.
Use property decorators for simple modifications within the model definition.
Use a custom data type if you need to perform complex pre-processing specific to your data type.

sqlalchemy