Question

is it possible in SQLAlchemy to enforce maximum string length of value assigned to mapped column? All I want is to raise an exception if an assigned string value is longer then the length of the corresponding table column of type STRING.

Thank you

Answer 1

It’s easiest to just rename the mapped column and proxy it through a property:

class Something(Base):
    ...
    _foo = Column('foo', String(123))

    @property
    def foo(self):
        return self._foo

    @foo.setter
    def foo(self, value):
        if len(value) > _foo.type.length:
            raise Exception("Value too long")
        self._foo = value 

You can easily factor out the property creation, and even use a generic validation framework like formencode.

---

If you need a more SQLAlchemy specific solution and don’t mind using specific interfaces, then SQLAlchemy has an extension mechanism for capturing events on attributes. A validator using that would look something like this:

from sqlalchemy.orm.interfaces import AttributeExtension, InstrumentationManager
from sqlalchemy.orm import ColumnProperty

class InstallValidatorListeners(InstrumentationManager):
    def post_configure_attribute(self, class_, key, inst):
        """Add validators for any attributes that can be validated."""
        prop = inst.prop
        # Only interested in simple columns, not relations
        if isinstance(prop, ColumnProperty) and len(prop.columns) == 1:
            col = prop.columns[0]
            # if we have string column with a length, install a length validator
            if isinstance(col.type, String) and col.type.length:
                inst.impl.extensions.insert(0, LengthValidator(col.type.length))

class ValidationError(Exception):
    pass

class LengthValidator(AttributeExtension):
    def __init__(self, max_length):
        self.max_length = max_length

    def set(self, state, value, oldvalue, initiator):
        if len(value) > self.max_length:
            raise ValidationError("Length %d exceeds allowed %d" %
                                (len(value), self.max_length))
        return value

You would then use this extension by setting __sa_instrumentation_manager__ = InstallValidatorListeners on any class you want validated. You can also just set it on the Base class if you want it to apply to all classes derived from it.